M1 Pro is 251.3 mm² with a ~5 TFLOPS GPU and Rembrandt is 210 mm² for 3.4 TFLOPS so the comparison checks out.

It's a slightly better comparison, however Ryzen 6000 is on N6 (which is an optimized N7 process), whereas M1 Max is on N5. N5 is considerably denser than N6.

AMD's next round of APUs on 5 nm will offer a better die area comparison.

I remember reading that at 5nm, electron tunnelling through transistor gates would be a major problem. What happened to that?

5nm doesn’t mean anything is actually 5nm. It’s the “5nm node”

Marketing dept has trumped physics.

That would be the case if 5nm didn't refer to a random feature of a reference planar transistor that is only used to chronologically name the process according to its mathematically equivalent density..

I think the do funky stuff with the 3D or 2.5D geometry of the gate so that the surface area (between transistors) is larger for a smaller footprint.

Someone correct me if I’m wrong!

Yes, that tech is called FinFET and it has been in use for many years. The new hot thing is GAAFET (Gate-all-around FET).