If the body was a heatsink, it would be extremely hot to the touch.

From your link:

Essentially the bottom cover of the MacBook Air becomes one large heatsink

Anyway, the author claims:

you are the type that likes to work with the MacBook Air on your lap it will be quite a bit more toasty than before.

Does toasty mean extremely hot?

The Apple M4 CPU is, if I recall correctly, capable of converting 20 watts of electrical energy in to heat, at full throttle.

Is that likely to bring the back plate or a MBA above 45 degrees?

You’re probably right, with sustained workloads it could.

Everything’s a trade off.

	▲	necovek 3 hours ago \| parent [-]
		In a Notebookcheck test, they got the bottom plate up to 43C, and top plate near the screen up to 45C: https://www.notebookcheck.net/The-passively-cooled-M4-SoC-ma...