Brokers must often sell customers' positions if they have insufficient margin. Where did he say they wouldn't?

True, but that's for leveraged positions. So I wonder was that leveraged position or just plain "own stock" position? If the latter, I never heard about broker liquidating a position against the will of the client, when settlement of debt is not involved. Does it happen?

Edit: screenshot from reddit: https://pbs.twimg.com/media/Es1pb5KWMAE19CD?format=jpg&name=...

It applies to any margin position. This can just mean trading with unsettled funds. E.g. you buy a stock, sell it, then use the proceeds to buy another stock (or the same one) before two days have elapsed.

$118? That's daylight robbery. It must have been the lowest point of trading today.

In fact, when I look at the stock chart on Yahoo the lowest value $GME had was $126!

https://finance.yahoo.com/quote/GME?p=GME

Forced selling doesn't just happen arbitrarily, but precisely because the price is low. Long positions using borrowed or unsettled funds are similar to shorts in that they can be squeezed, just in the other direction.