Again, this is not locative. It is prepositional. The locative form would be "на стулУ"
Understood, but in some English-language textbooks about Russian grammar, the terms "locative" and "prepositional" are used interchangeably in reference to "ком/чём", even though properly they should be distinguished, as you said. That's why I wrote "locative/prepositional", because I wasn't sure which term Astrum would be most familiar with.

P.S. In English textbooks that use "locative" and "prepositional" as synonyms (or that use "locative" instead of "prepositional"), forms like в саду, when they exist, are often referred to as "2nd locative", in my experience.

P.P.S. In my first-year college Russian textbook, I'm pretty sure (though it's been many years!) that the term "prepositional" wasn't used at all -- so, e.g., both в магазине and об американцах were described as examples of the "locative case", even though the second one has nothing at all to do with location! I would guess that this odd use is the result of trying to map traditional Latin grammar terms onto Russian.