Not at all revisionist. It doesn't redeem him in the slightest. Quite the reverse, in fact. The theory revolves around Kennedy and Mary Jo being spotted (At or after 12:30AM, some 60-75 minutes after they were seen leaving the "Boiler Room") in a local lover's lane by a local sheriff's deputy. They take off, leaving the deputy behind. At this time, the following facts exist, but Kennedy doesn't know any of them:
The deputy did not get the make and model of the car, the license plate # or state of origin. He recognized no one in the car before it sped off. He had no access to any form of transportation nor communications.
Though Kennedy did not know it, he had already made a clean getaway. But he didn't know that, and that was where the wheels came off the wagon.
He had to assume the absolute worst:
That the deputy got his license plate number, recognized his face, was calling for backup, was racing to his police cruiser, and that Kennedy would run right into an incoming police cruiser coming onto the island.
So, what to do? Remember, both he and Mary Jo are highly intoxicated at this point.
"Mary Jo, I want you to take the car. When they stop you, just tell them I loaned you the car, and you were going home alone by yourself. I'll just tell anyone who asks that I'm taking a walk to clear my head..."
So she leaves, and Kennedy walks on. Mary Jo, very drunk, driving an unfamiliar vehicle, on a darkened bridge with no guardrail, slips over the side on the driver's side forward wheel. It skids a short distance, and the car goes over, landing in the water bellyflopper style, with the shock of impact transmitting to the driver Mary Jo, knocking her unconscious. She wakes up as the water fills the car, and she gets to the air pocket at the rear window, quickly using up the air there and asphyxiating.
We cannot know if she knew in her intoxicated state whether she could ever have known that the car was only barely submerged. Also if she could have had the strength to open any of the windows or doors.
In the dark of the night, the drunken Kennedy walks across the bridge, seeing nothing. He then checks into the motel, being recognized by the night clerk. Kennedy then proceeds to chat up the night clerk, going back and forth to and from his room, chatting with the clerk.
Establishing an alibi for his whereabouts, should Mary Jo be stopped by the police. Finally, at 3AM, he goes to bed for the night.
At about 7:30AM, he gets up and goes outside. He sees police, fire, and emergency vehicles. But the first person HE runs into is one of his own aides. What happened next, until the press conference, we will probably never know.
The following facts ARE in evidence, however:
Kennedy did not arrive at the motel drenched in mud and water. He was at the press conference in the same clothes he had on the night before. They were still clean. His insipid web of lies made sense ONLY if you are trying to cover up the FIRST set of lies he had prepared to cover up an affair.
Why? Because in the most cynical of political calculations, Kennedy knew then and there that his Presidential future was over (though it took his disastrous 1980 nomination campaign to convince him once and for all). That press conference was all about saving his Senate career, his political future in Massachusetts. The people in that state were prepared to believe any story he had to tell as long as it didn't involve Nazis on the Moon!

Good line.
zoomar
The trick is to try to get your head around the nature of the times.
It was 1969. DUI, even involving the deaths of innocents, was not looked upon as a crime by society then. Only as rank foolishness. The worst you'd be looking at, most of the time, is a lifetime suspension of your driver's license.
But for politicians,
caught with another woman, that was political suicide. Today, the wheel has come full circle. The only way a politician can be destroyed by an affair today is if he is a "family values" politician.
So pleading to the "lesser offense" of an affair? Politically it WASN'T the lesser offense. And beyond that, there was having to admit that he set up a deliberate web of lies to coverup the affair, leading up to Mary Jo's death. I wonder if even the voters of Massachusetts would be able to swallow THAT!
Kennedy, once sober, knew what he was doing.
You know how the truth is always stranger than fiction? Besides, there were very powerful political forces in play who would have been delighted to shred Kennedy had he told the truth (paging Nixon Administration).
How many times do people have to tell a lie, no matter how silly, because the truth would never be believed? Remember, Kennedy lied about the timing of the accident, and called the deputy a liar. Kennedy was hoisted up his own petard.
The deputy said: "Look, you can believe me, or you can believe him. Take your choice!" Apparently, the people chose to believe him. They elected him Sheriff.

The trick is, don't allow yourself to cherrypick the data. If the deputy is telling the truth, then Kennedy is lying about the time of the accident. If he's lying about that, he can be reliably considered to be lying about everything else.
(*) Not "A reason to exist." Just a perfect example of the old chestnut of "Oh, what a tangled web we weave, when first we practice to deceive."