From your first link, it seems the decision to use a different new domain stems from difficulties getting the server's HSTS policy right, and it even seems they had a similar issue in the past with having the store as a subdomain [1].
If that's true, for a use case as functionally basic as having a store and a social instance in their respective subdomains, it looks to me like a complete failure of HSTS, a case of technology causing problems that shouldn't exist to begin with.