I think Lemmy has a problem with history in general, since most people on here have degrees/training in STEM. I see a lot of inaccurate “pop history” shared on here, and a lack of understanding of historiography/how historians analyze primary sources.
The rejection of Jesus’s historicity seems to be accepting C S Lewis’s argument - that if he existed, he was a “lunatic, liar, or lord,” instead of realizing that there was nothing unusual about a messianic Jewish troublemaker in Judea during the early Roman Empire.
What I remember from Bible Geek (and/or Human Bible, another podcast he did) was that the earliest of the gospels actually dates to the 4th century CE, and that three of them are likely derivative works from an earlier book, lost to us, that scholars call “Q.” I think it was John that was the only gospel thought not to originate from it.
Addition: looking it up, here’s Q source on Wikipedia. It states that Matthew and Luke are thought to originate from Q, but not Mark or John.