By this measure, any historical fiction that features fictional characters that did not actually exist is "essentially alt-history". That so dilutes the concept so as to make it meaningless and absurd.
When I read an Horatio Hornblower or Jack Aubry book, I do not read it as alternate history. I would expect and demand that the Napoleonic wars end as they did in our TL and that actual historical figures that appear be as true to what we know from the historic record as possible.
Well, to pick a nonnatutical example, the Sharpe books. I would say a fictional regiment which is involved as that one is definitely counts as bordering on alternate history.
I have to admit I know more about Hornblower's fictional career than Aubrey's, but I'd say having a fictional high ranking officer (the protagonist over the course of their career) who does important stuff is more than just "historical fiction with a fictional character". It would be like if Kilrain was a general instead of a sergeant.
In any case, all of those are historically sound and entertaining, so it would be entirely possible to write something in a "What if the Lost Orders were never lost? What if Lee won the Antietam campaign?" world that was plausible and a good read, which was what I was trying (awkwardly, I know) to say.
That's the test of a fantasy world, without any resemblance to ours. Alternate history is supposed to be alternate history, not fantasy (and I say this as someone who lists The Last Unicorn as one of their favorite books, so I have nothing against well written fantasy). If you claim this is our world after X has changed, then it should reflect the results OF X.When, as in How Few Remain, I read a book set some 20 years after a PoD in the civil war featuring a Samuel Clemens who never became Mark Twain, an independent Confederacy, and a host of other historical figures who have diverged from our TL, I judge plausbility, not against our history, but against the internal logic of the alternate history being presented. At least in the first few novels of the TL-191 series, I consider the internal logic quite plausible.
So while I don't know how internally logical the TL-191 series is, I would expect more of the author than "absurd parallels even when things have changed so much that they wouldn't happen" and similar crap because actually writing a plausible alternate history world to set his novels in would be work.