I can't tell who is selling but I can tell: a) while there did seem to be a modest improvement in the power-up/power-down balance (suggesting less selling on net) after EIP/HF21, it didn't grow beyond modest and seems to have been a small one step improvement at best (if sustained, which isn't even clear). b) There is still a shit load of tokens going out to exchanges every day and we dropped another 50% in price despite Steemit Inc taking a temporary break from selling.

Those earning significant rewards can only sell once

If only that were true. They can crank out more cookie-cutter content and get more new coins to sell over and over again every single day.