Not to rain on his parade but he is just getting lucky. Sports betting is hard and a simple strategy like that isn't going to yield long-term results.
The ERAs will stabilize, "regress to the mean" if you will, per individual pitcher(s) as more games are played. Early season one bad outing can ballooon a guy's ERA of course (don't need to tell you that lol), "Under" or "Over" umps, Weather, Team Trends (Boston scoring 51 runs over the course of 4 games...KC scoring 11 over the course of 6 games etc.)...so many reasons that this should not be working but it is.
Early Season Phenomenon?
Many seasoned Bettors do say: "Let the Linesmakers tell you what to bet".
Seems like that is whats going on here. And its working. At least for now.