They will of course be at the coronation.
If they weren't invited (an apparently they are), they'd have made a big victim song and dance about it. In public - not "the media", but in ways "the media" would be talking about.
Inviting them keeps them quiet about not being invited.
Not going after being invited erodes their victim status.
Coming to the coronation keeps their profile up, plus gives opportunity to be at a major historical event. And gives them the opportunity to have another series/book next year/2024. If they choose. The only way they can continue being victims of the RF is, ironically, by attending. Unless they have a radical other business model, or have earned so much they don't need to earn any more, then the victimhood status is important.
But the other thing is that by going to "support" the King, they make themselves look like bigger twats to many. And I wonder actually if this is the real reason their invitation is public. H&M are playing chess with people who can plan ahead. Something they've not shown themselves skilled at (yet).