By syn, they meant equivalent. I'm sure it's technically not difficult to add characters A and B if the work is tagged canonical and fully wrangled A/B, but unlike you, they (and I) don't think that should be intended behavior because relationship A/B should not equal characters A and B. Plenty of fic exist where A/B is an appropriate tag and character A and/or character B less so.
Re: Unpopular Opinions About Changing Things
By syn, they meant equivalent. I'm sure it's technically not difficult to add characters A and B if the work is tagged canonical and fully wrangled A/B, but unlike you, they (and I) don't think that should be intended behavior because relationship A/B should not equal characters A and B. Plenty of fic exist where A/B is an appropriate tag and character A and/or character B less so.