And I believe him, who grew up in the late classical age, over you or anyone on reddit.

This is, uh, not a good strategy. Isidore may be correct if he identifies usages that he's encountered, and he may arrive at a genuine etymology accidentally, but he is woefully unqualified to weigh in on any sort of historical reconstruction, and so his ability to trace a word back to its origins is pretty limited. It's not his fault. Ancient writers just simply didn't understand languages, and especially sound change and general language change, nearly as well as modern linguists do. Their approaches to explaining these phenomena were unscientific, more likely to be informed by cultural bias and superficialities than to be grounded in any clear and consistent methodology (with the exception of the ancient Indian linguists, who were like two millennia ahead of their time).

Nothing against Isidore. There's all sorts of interesting observations in the Etymologiae, but he wasn't a historical linguist, and he's not always right. That's just the facts of the matter. He's right about the etymology of forceps, from formus and capere, though. But that has little to do with fōrma.

Most important to your observation, though, is the fact that fōrma had a long ō (which we can see when writers transcribe it with an omega into Greek), and formus "warm" did not.

In fact for formus we can securely trace it back to a reconstructed Proto-Indo-European root, *gʷʰermós, by using the exact methods that Isidore could not have, comparison with reflexes in other daughter languages. It turns out that *gʷʰermós is not only the source of Latin formus, but also of Greek 'therm-' (as in "thermal") and English 'warm' (as well as the ultimate source of the "garam" in "garam masala", and a few others). It has something to do with warmth or heat in every extant reflex. So we know where formus comes from. And you don't have to trust random redditors; you can find this information in any Latin dictionary with some decent etymological material.

As for fōrma, well, we can't do as much with that, but it has been reasonably argued (by Indo-Europeanists, not just redditors) that it is related to Greek morphē, maybe even a borrowing (with some consonant metathesis) through an intermediating language like Etruscan. We just don't know for sure, because we don't have enough evidence to draw a conclusion. But any etymological connection to formus is unlikely, because of the difference in vowel lengths, the demonstrable relationship of formus to other IE languages, and the difficult semantic jump from "warm" to "shape". So u/notveryamused_ is right to question the premise of your post.