Because you presented something as true (with the link) which you also believe is true, which is not.
First, this isn't some "Stormwind Fallacy." This is just someone trying to argue that this is a false dichotomy (sometimes referred to as a false dilemma, but in this case, it's really a false dichotomy).
Nod. In that sense, 'Stormwind Fallacy' is just a cute hobby-specific label for the excluded-middle/false-dilemma/etc informal fallacy.
Just someone putting fluff first. ;P
Argument-
You have to be either a roleplayer, or an optimizer.
(You must be A, or B.)
The premise is false, because you it presents two opposite, when there are other avenues possible.
....People don't say that the two are always mutually exclusive; that you cannot engage in any roleplaying if you optimize, or that you cannot care about the rules at all if roleplay; instead, it's a question of what emphasis you put on the game. Because an emphasis on optimization and rules will eventually come at the expense of roleplaying, and vice versa.
By creating that post, and the fallacious fallacy, the poster was engaged in some rhetorical slight of hand. Which is deeply annoying.
I think I see some rhetorical slight of hand, in that bolded bit. Aren't you just coming back around to asserting that two perfectly compatible things - 'roleplaying'* and 'optimization' are incompatible by nature? That they must conflict, even if only 'eventually?'
The acid test is:
Would the player take a sub-optimal option/make a sub-optimal build choice in favour of adding fluff/flavour to their character.
Or, to put it another way, is the player convinced that the "optimal" build choice can never be taken for fluff/flavor reasons...
;P
You can just not care about fluff/flavor and make choices based only on some mechanical goal, and make ridiculous fluff choices.
You can be totally into the minutia of your character's fluff, and make terrible build choices.
You can have a very definite concept, and make choices to optimally model that concept mechanically and thus realize it in play to the fullest extent possible in the system.
The better the design of the system - the more intuitive, balanced, consistent, clear & flavorful the game - the closer the character produced by each of those three approaches will be to the others. In a hypothetical 'perfect' system, all three attitudes could yield the exact same character.
* another big, bright, violently-waving red flag in any of these false dichotomies is if the extreme on one side of the excluded middle gets labeled 'Roleplaying.' It's in the context of an RPG, it's all roleplaying.