From a US point of view maybe but it’s complicated in Japan. For starters you are not allowed to work remotely on a tourist visa waiver and work done remotely I Japan is not “foreign sourced income”, at least not for a normal situation. There’s conditions which may trigger tax residency, and finally regardless of your status, work done in Japan can cause liabilities for the US company.

Also the posts I mentioned are almost universally “I assume or have assumed that I can just buy a house and work remotely from Japan”, but there a few that are looking for loopholes to be a digital nomad.

You are most likely correct - those who want to own a property/live there forever are out of luck and likely bend local laws. It's always best to talk to your tax advisor wrt these issues to avoid any troubles later (I did to mine before I traveled around the world working remotely).

You still need residency for your income. In this case, it'll probably be the USA unless you can convince the IRS otherwise.