Or they were but it was kept secret. Twitter hasn't published a list, we only know of the BTC tweets. Maybe they actually were after other accounts' DMs and the tweets are just diversion to make it seem like an undirected attack.
Unless we hear from account holders that their credentials weren't stolen, there's no reason to believe that only those were hacked that sent tweets.
Except that is all the evidence we have to go on for this conversation. Verified fake tweets have been sent from prominent democrats, and not from any prominent republicans.
Of course you're right that we don't know is if this is political, or just a distraction from whatever their real goal is / was. But the optics are clear here, and there is no reason to muddy the waters.
Unless we hear from account holders that their credentials weren't stolen, there's no reason to believe that only those were hacked that sent tweets.