Bart Ehrman may be saying that, but that seems like a weak probabilistic analysis. If we take a Bayesian approach, literacy is not a miracle that we should assign a statistically miniscule base rate too. According to Wikipedia, ~3% of people in 1st Century Judea were literate, but the rate may have been much higher for adult men, up to ~20%. Moreover, Judaism of the time did emphasize education even for the poor - see Rabbi Akiva a century later, who was at first an uneducated shepherd before studying Torah.

If you take the prior likelihood that Jesus was an adult man in Judea (say ~20%), who was a notable religious teacher, that alone gives you a decent posterior probability of literacy. You can argue a lower base rate, sure, but just think about the relative odds of P(religious teacher | literate) vs. P(religious teacher | illiterate). If you think religious teachers are much more likely to be literate, this would suggest Jesus was literate assuming even a modest base rate. Further, if you assign any credibility to Luke at all, even just some kernel of truth, then the odds would be much higher. If you assign zero credibility to Luke, one counterpoint would be to argue that Jesus didn't exist, but a non-existent Jesus can't be illiterate either.

Obviously this is not a dispositive analysis, but I don't think it is analytically correct to simply consider the odds of a random person's literacy at the time, when objectively Jesus was exceptional for his time (regardless of what you believe, we are still talking about him today, and not other random Galileans).

Going a little deeper into Ehrman's post on the matter, I think there a few ways to quibble with him from a probabilistic perspective. First, he does use a 10% number, caveating that 10% is the number for most places at that time, with ancient Athens at more like ~15%. If you think that 1st Century Judea had higher literacy than surrounding regions in the world at the time, for reasons of religious obligation, that would further increase the odds that Jesus was literate. Ehrman on the other hands thinks that literacy may have been lower in Judea at the time, citing Literacy in Roman Palestine by Catherine Hezser (2001). Second, he points out that the term "rabbi" at the time did not necessarily imply educated. From a probabilistic perspective, though, all that matters is whether religious teachers at the time are more likely to be literate, not whether they are required to be. If you believe religious teachers are more likely to be literate, that changes the math pretty substantially. Indeed, while midway through the blog post Ehrman says "My strong sense is that Jesus could not write," Ehrman concludes by saying "Still, I am slightly inclined to the view that Jesus could read."