It's not, because Apple purposefully lied on their marketing material. Letting a 3090 go on full blast brings it pretty much in line in perf/watt. Your 3090 will not massively thermal throttle after 30 minutes either, but the M1 Ultra will.

So, yes, if you want to look good on pointless benchmarks, a M1 ultra ran for 1 minute is more efficient than a downclocked 3090.