Commenter was referring to its youth in the early 00s, and by that time mobile phones were ubiquitous and thanks to carrier subsidizing also pretty affordable.

The mentioned (preinstalled) app functions already came up during the switch away from simple text based displays in the second half of the 90s (as well as the texting you mentioned).

You are partly right, though, before that time items where more of a "single purpose" type.
But even then, miniaturization and mass-production had already rendered the exact "You won't have a calculator in your pocket all the time" phrase nonsensical, with everybody typically having lying around several of the free tiny giveaway solar-powered calculators in bags and drawers.

I would guess that the original phrases the potential collective false memory is based upon, were something more like "you won't be able to use a calculator in every situation", which then got transmogrified over time by social reinforcement to the plausible sounding and much more exciting "You won't have a calculator in your pocket all the time" trope.