Because the declaration file of the clients says that it is actually compatible with everything in this section.
You can't run scripts on all the XEPs declared, some of them are purely redaction or bound to specific UI/UX behaviors. This is based on trust that the developers actually implemented things as stated.
> I just wish there was some kind of ACID test suite for XMPP or something else to reproducibly validate spec compliance. Maybe a test server or similar as a reference implementation. This way client or server maintainers would have to run their programs against the official test server to increase their compliance stats.
This is exactly what the Compliance Suits are for, and the XMPP Software Fundation is taking care of telling all the clients what they misses directly on the official website, for example: https://xmpp.org/software/movim/