It seems like he made clear in the annual letter that the cash levels and not buying all that much during the Q4 drop was a market call. Something about the value of high quality businesses being "sky high". Perhaps that's just saving face, though.
If the market is 20-25% lower in 2020 then, no, I don't think this was Buffett's AT&T moment. If things just go churning on upwards and Berkshire underperforms the S&P by a few points each year, then yeah.
Basically, this could be an AT&T moment, but who can tell without hindsight.
About 40 or 50% of my portfolio has been in Berkshire for a while... just for context/disclosure.