I meant we don't know what other information they can see. Whether a private header conflicts with the cartridge ID, etc.
Besides, have you considered that if they start banning people for using private headers and .CIA files online, we'll all be forced to only play offline anyway? Whether you're doing it to avoid a ban or because you're already banned won't matter at that point. So you might as well get in what online play you can in the meantime, should you expect that to be an inevitability.
I didn't say private headers are guaranteed safe for all time. I said they're safe now; they're safe until they aren't.