I'd call it being uncharitable. Extremely so, in this case.

Salviati: blah blah blah Exercise increases lifespan blah blah blah

Simplicio: THAT'S NOT TRUE THERE EXISTS AN EXCEPTION YOUR ENTIRE ARGUMENT IS INVALID

Because we're talking about being uncharitable, let's be charitable for a moment. Simplicio, in fact, made the mathematically proper counterargument: he produced a counterexample to a for-all claim. And finding one flaw with a mathematical proof is, in fact, sufficient to disregard the entire thing.

Clearly, though, Simplicio's argument is horrible and nobody should ever make it. If we check out the errata for Linear Algebra Done Right, we find that Dr. Axler derped some coefficients on page 81. His proof is incorrect, but any reasonable person can easily see how the coefficients were derped and what the correct coefficients were, and it's a trivial matter to change the proof to a correct proof.

Analogously, the proper response to an argument that's technically incorrect, but has an obvious correct argument that you know the author was making even if they phrased it poorly, is to replace the incorrect argument with the correct argument, not scream about the incorrect argument. Anyone who does anything differently should have their internet privileges revoked. It's more than a trivial inconvenience to write (and read) "the overwhelming scientific consensus indicates that, for most individuals, increasing exercise increases lifespan, although there's a few studies that may suggest the opposite, and there's a few outliers for whom increased exercise reduces lifespan" instead of "exercise increases lifespan".

So, now our argument looks like

Salviati: blah blah blah Exercise increases lifespan blah blah blah

Simplicio: THAT'S NOT TRUE THERE EXISTS AN EXCEPTION YOUR ENTIRE ARGUMENT IS INVALID

Salviati: Principle of charity, bro

Now, if Simplicio applies principle of charity, then they'll never make arguments like that again, and we've resolved the problem. If they don't, we discontinue debating with them, and we've resolved the problem.

There's a few failure modes here. We create a new route down which debates about akrasia-fighting devices can be derailed. We give a superweapon to people who we probably shouldn't trust with one. They may google it and find our community and we won't be able to keep them out of our walled garden. (I jest. Well, maybe.) But introducing principle of charity to people who have clearly never heard of it feels like it should either improve the quality of discourse or identify places we don't want to spend any time.