That is the key question, I think - especially since the people trading Bitcoin for USDT would've been doing so at prices which value USDT for slightly above fa

That is the key question, I think - especially since the people trading Bitcoin for USDT would've been doing so at prices which value USDT for slightly above face value. (That is, on all but a handful of occasions they could've got more by selling their Bitcoin for actual USD on Coinbase. The main exceptions were after the Tether hack and during this recent bout of negative news coverage, if I remember correctly.)