It's still a huge anomaly, and while staying home may in-fact activate some to vote via mail, it may also passivate others to not vote at all. I don't find it feasible for the voter-base to suddenly bloat 20% just because of the ability to vote by mail. (We get a lot of gallops and other stuff via mail, that we never even open.) Also, shouldn't it reflect in the walk-in votes too? There should be a negative correlation, because currently, it looks like there's almost as many extra walk-ins as there were mail-in voters. The mail-ins should have eaten away participation from the walk-ins, at least a bit.
To me it looks like the actual voter-base suddenly got a lot bigger for one Election only.