Sure, I understand that, but this user is claiming that on GCP even local SSDs aren't really local, which raises the question of why not.

I suspect the answer is something to do with their manufacturing processes/rack designs. When I worked there (pre GCP) machines had only a tiny disk used for booting and they wanted to get rid of that. Storage was handled by "diskful" machines that had dedicated trays of HDDs connected to their motherboards. If your datacenters and manufacturing processes are optimized for building machines that are either compute or storage but not both, perhaps the more normal cloud model is hard to support and that pushes you towards trying to aggregate storage even for "local" SSD or something.