I dont think the USD reserve was to prevent them from selling BTC. Its to amass dividend payments for their preferred stocks (STRK, etc).

People were concerned that they weren't going to be able to meet monthly dividend demand. So this is an attempt to assure investors by keeping >12months of divis "in stock".

Technically MSTR wouldn't be triggered into selling BTC until 2027 and if BTC drops ~50% at that time.