is my instinct here outdated/inapplicable?

Yes.

It’s so cheap to send SMS messages, and you don’t pay for undeliverable messages, so they can just send to random numbers.

They also receive deliverability responses for each number. So they know whether a phone received the message whether or not you reply.

Finally, if you reply STOP you’re unlikely to fit their demographic very well anyway. As in… they’re not trying to reach the type of people who will actively try to avoid receiving these messages.

That said, there’s probably no point replying STOP because most firms just wont honor it in the long term. As in they might not message you for the remainder of that particular messaging project (campaign), but they’ll just start a new campaign tomorrow with a new sender and no “STOP” requests.