Is it highly unethical? You're hired to do a job, if you can successfully do that job and another one, I don't think there is anything unethical about doing that. It's very common in lower wage positions, not sure why it's suddenly unethical in higher wage positions. I would say it's unethical to force people to sign a contract saying you own all of their attention during the day, even if you don't use it.

If you sign a contract, then it is breach of contract, which would be a kind of illegal.

I don't see the ethical quandaries though. So long as one is doing the work promised, and the company is satisfied with their output. It's not any more unethical than working on any other side project after hours.

It's true, I know the guy. He's an outlier for sure, but he exists. Moreover, he moved from the toughest FAANG to more relaxed ones.

I personally know I person who works two remote PM jobs at the same time and in addition runs her own side gig. Sometimes she joins two meetings from the two laptops at the same time.

I know a guy who overlapped months of two different jobs.

So it exists and happens. And for IC-type positions should be even easier.