With total and absolute state repression of Christianity when it was underground completely failing, how exactly are milquetoast efforts going to stop an ever-growing religion that comprised more than half the population of the Empire by the time of the reign of the Apostate?
Because Julian wasn't repressing Christianity. There was no persecution of Christians, no outlawing of Christianity, no repression. Hell, the head of his imperial guard was a Christian (the future emperor Jovian). His policy was essentially to remove state support from Christianity and promote pagan religions. So, for example, by instituting an edict proclaiming religious tolerance, he made all the Christian heresies legal which was intended to start infighting among the Christian groups like there'd always been. He showed favor to non-Christians over Christians, which over time would make many of the upper class abandon Christianity, for the very same reasons they adopted Christianity-because it would give them a better chance of getting positions within the bureacracy. The same goes for his policy of banning Christians teaching classical works (which was probably the only kind of repression he enacted AFAIK). For a wealthy Roman family, being taught the classics was an integral part of their education in preparing them for a public service career. Therefore families would have to choose between finding a good Christian teacher and sacraficing that vital part of their education, and finding a pagan teacher so they could still get that education-most upper class families would almost certainly choose the latter.
And then of course, over time he would likely be promoting some kind of pagan cult, such as Sol Invictus (for example) as an alternative to Christianity. The result of course was almost no effect in his short 18 month reign because these were longterm policies. Of course, if he
was merely persecuting Christians, it would be a completely different story and you would be correct, but that's not what he was doing at all. He recognized how much of a failure those kinds of policies were, and instead took a different route.
Because none of the repression of those times had viable long term strategies. It was reaction and not action. Julian had the devious idea to use the beraucracy against the christians, which while having little to no effect in the short term, would gain effectiveness over time as Christians failed to gain position in society.
It wasn't completely failing. Had it been sustained indefinitely it may have even worked. Regardless I don't think Julian's long term game was the suppression of Christianity, merely marginalizing it into a minor religion completely cut off from temporal power and wealth.
Essentially this.