That seems like a tough road. They've never been able to enforce that iPhone app makers build an iPad-compatible version or vice versa, and this looks exactly like that scenario.

It's mostly enforced, the only exception being apps with a legitimate iPhone-only hardware dependency.

iPhone apps must function on iPad in iPhone-compatibility mode, apps must support window resizability as of OS 27, and apps can't detect iPad and refuse to run. And I don't think this is new, but setting the target to iPhone-only doesn't prevent iPhone apps from running on iPad.