I did a quick Google.
https://web.eece.maine.edu/~vweaver/group/green_machines.html

Is the best actual test data I can find. It uses a physical power meter, so it's full system (not TDP or self reporting power consumption).
And it's a few years out of date.
Seems like Apple silicon is the winner (and will probably continue to be).
The Xeon that beats the rpi4 for GFLOPS/watt is an e5v3, which was launched in 2013 and EOL in 2021.
So there will absolutely be some new Xeon CPUs that will perform better.

However, for a $50 device, it's probably the best GFLOPS/watt/$ from what little empirical data I can find