...but even repeated soft bounces is a message level event, not one that means there will never be an opportunity to deliver to this address again. Hence Mailgun itself not adding this to their permanent uppression list..but that implies, right, that they will send to the permanent failure hook in this case?
That could be a problem, if it actually send to the permanent failure hook in this case. Then you would have to hit their bounces API to check whether it's actually a permanent failure / hard bounce for the recipient as opposed to just for this message.