This is probably a productivity boost for engineers who would have been sitting nearby

Of course. Sales people make money. They are ones who put money in your wallet. The good ones are the last people you want to scare off. Even worse is that they can be scared away to a competitor.

Developers, on the other hand, are a replaceable cost center. Developers never make money. They only cost money, and if a developer gets upset they can be replaces with minimal consequence.

This seems a little bananas, since I assumed RTO was always a sales thing the rest of us had to live with.

Many outside sales people have worked from home for quite a while. My wife worked as a scientific sales person and worked from home. The home office was on the other side of the US and she was responsible for her territory.