Laws don't have to be enforced, but it's still the law. You asked for a law, and REAL ID is one. Although it isn't fully enforced, the provision for an ID is fully enforced. It's just not required to be a REAL ID, but an ID is required.
Which provision, specifically, are you referring to? The REAL ID pdf you linked does not contain a "provision for an ID" in airports separate from 202(a)(1), which is not yet in effect, and will likely get snoozed for a few years again in 2025. Furthermore, even if it were in effect today, it would not require the checking of IDs; it merely prohibits the checking of non-REAL IDs.