> That's why the founder can threaten them with the IRS, because they (likely(1)) do not rightfully (as defined per law) own it.

This statement is nonsensical - the IRS has no enforcement mandate for theft/fraud, nor does it have any authority to return stolen property. All the IRS would do is say "Hey, I heard you got an extra $NN last year - please pay your taxes on it".

It could even be argued that by reporting people the the IRS, the founder is implicitly admitting that the current possessors actually do own said crypto, otherwise they wouldn't owe any tax on it.