But that's exactly it! It ISN'T a "them" problem! Even if a user owns their own instance and digitally signs every post, a bridge NECESSARILY modifies the posts it bridges, thereby invalidating that signature.
If a protocol requires everybody ELSE to bend over backwards to accomodate it then that protocol doesn't play nicely with others! There's no point in sugarcoating it!
I wonder, does Pubky have this problem federating with ActivityPub?