To me this (from the linked policy)…
Third-and-higher-level domain names within a registered second level domain may
only be used internally by the registrant of the second level domain
…means members of meet.coop should be able to use subdomain of meet.coop - it is internal use.
I guess the issue is that web architects are the owners of meet.coop and members of meet.coop aren’t internal to web architects.
I don’t think this would be an issue if the meet.coop domain were owned by meet.coop the co-op.
Can unincorporated co-ops own domains?