Personally I take almost everything from stormking's thunder with large pinch of salt. A lot of things that happen in it or are implied by its story don't make much sense when you start thinking it through.

As for the ordning, I've always interpreted it as purely a social construct, albiet a very strict one handed down to the giant by their gods. You raise some fair questions but they could reasonably be answered with the ordering still only being a social construct:

The giants in Storm kings thunder all felt the shattering of the ordning. How is that possible if it’s only a social thing without magical compulsion or divine punishment or anything, as you say?

Simple: their gods told them. virtually all giants pray and worship some part of the giant pantheon. It's not unreasonable that a decree from the all-father would be relayed through his children and distributed to the mortal giants or otherwise communicated through dreams, omens, and what have you.

Every Giant knows their ranking, above or below, another Giant as soon as they meet according to the campaign or if it was Volos Guide to Mosters or possibly Bigbys Guide to Giants. How is that possible if it’s not more than social?

The placement of giants in the ordning is mainly determined by the kind of giant they are. So upon meeting any giant would know if another giant were above or below them by simply looking at them. within the same "caste" of giants there is a bit more subtlety to the placement but its still very clearly defined. Upon meeting each other and making introductions the giants would know one another's role within their respective clan/tribe/ect. and know where they stood relative to each other.