There are several video recordings of them acknowledging that the stake was intended to be non-voting and spent on development. Making that statement does hold legal obligations.
And you can't buy a stolen car and expect to keep it after the police come to your door. Justin Sun may not have known about the obligations put on the stake, but those obligations were made by the CEO and CTO of Steemit Inc., which means that Steemit Inc., which is the actual holder of the stake continues obligated to fulfill the statements.