XMP is technically OC.

The 12th gen support DDR4-3200 and DDR5-4800, but these are JEDEC profiles. XMP is never guaranteed to work, and whether it's likely to work or not is normally determined by looking at various reviews. If it works for most people, then it's likely to work for you. Which is why getting XMP DDR5 kits is a bad idea at this moment: it's simply too unpopular yet, not mature enough to make any estimation on whether XMP works or not.

Even most DDR4-3200 are technically OC because they usually have latency around CL16 when the fastest JEDEC profile for DDR4-3200 is CL20, I believe.

And I've read a few times that the 12th gen has issues with XMP in general, even at lower DDR4 speeds. Just google for something like Z690 XMP issues and you'll see a lot of them. It still works fine for many people, though, at least in the 3200 CL16 range.

Yes, and 10900k only supports DDR4-2933.