Svlad Cjelli It's a part of the East half of the South Stand. If that half of the stand is sold first (and only that initially sold) then it's quite possible that the whole of that half of the stand is sold and tickets allocated there.
Later, when the other half (the West half of the South Stand) is put on sale then tickets there can be sold as a first priority.
I presume you weren't there on Saturday then?
Anyway, there were around 20 I'd say sat in that triangle - with around the same number of seats empty in that half of the south stand next to it. So, by my reckoning, they should have been sat in the main block in the South Stand, not in that triangle - knowing it was Cardiff - I'm surprised they would have even considered selling tickets in that wedge right next to Y26 and I'm pretty sure they didn't. The stewards just didn't bother to move them.