You're right, I've always misinterpreted the word, it's more neutral than I thought!

It comes from the Latin stolidus: “inert, dull, stupid”. So it's a strange case of a word with initially negative connotations that becomes somewhat neutral. Although Oxford Languages (Google's source) lists also "bovine", "lethargic", "torpid" and "stupid" as similar words, after "impassive", "unemotional" and "calm".