EDIT: Sorry, I was tired when I posted this, and didn’t realise that it made no sense to anyone else. I’ve got a set of panniers for the bike that attach under the seat, and they hold the smaller things, like my clothes and toiletries.
I remembered after I’d already attached my camping equipment to the top of the seat, meaning that it would have to be removed to fit the other luggage.
Clearly his luggage that connects under his seat is different than the luggage that’s already attached to underneath his seat. Or maybe he posted the “after” photo.