> Philo doesn't ever mention Jesus
Why would he? He lived in Alexandria and moved in different circles. And he's not dealing with contemporary events. He barely mentions anyone. His overwhelming focus is on philosophy.
The idea is not that Philo thought Jesus of Nazareth was the Logos. Far from it. He borrowed the word Logos from the Stoics, and used it instead of the word Demiurge to avoid a problem of overlapping terms with different meanings.
Carrier's idea is that Philo, as the outstanding Jewish philosopher of his day, probably had a following among Hellenized Jews who were anxious to understand Judaism within a Platonic framework. Philo is famous for making the claim that Plato and the Greeks borrowed their philosophy from the Jews, who got it from Moses.
It's Carrier's idea (but not original to him, I don't think) that Paul was a Hellenized Jew whose belief system before Christianity was Greco-Jewish syncretism. In other words, Platonism or Philonism. It's an old idea among historicists that Paul's conversion experience on the road to Damascus might have been a sudden insight that Jesus of Nazareth was Philo's Logos.
But mythicists have always objected that the account of Paul's conversion is only from Acts (say 90 CE).
Paul himself only tells us he had some kind of revelation at some point. Before that he was persecuting the Christians, and he himself was "was advancing in Judaism beyond many of my own age among my people and was extremely zealous for the traditions of my fathers." (Galatians 1:14).
So, Carrier doesn't get specific. Someone, could have been Paul himself, could have been someone else or some group, could have plausibly drawn on the ideas of Philo to create a Mythical Jesus, after which Jesus became historicized in tradition, and later in the gospels.
To my way of thinking, Paul would certainly be the prime suspect for this plot line but you've already seen how I avoid coming to final conclusions. It's my Libra Rising ;)