Your problem Kammer, is that you're always right.
The question is, did anyone think anything of it before old Walter Scott brought it to people's attention? He was certainly aware of all sorts of folklore, he was always incorporating it into his books. So I'm sure it had a reputation before he turned up?? Or was it part of the tourist trade for the Spa, and entirely spurious? But if it's called the popping stone, and if that's (as Kentigern points out) from the OE poppel - pebblestone, maybe that suggests an earlier recognition??
Oh I dunno.