It applies to the big hitters vs boxer matchups. It's finishing ability that counts, not punching power and that's a main reason why the ko guys lose that big ko % when they have to fight top 10 competition. Elite competition doesn't get that why by having a history of getting ko'd, as does the C and B grade fighters.
It certainly isn't a fallacy, depending on the match-up. Two good forthcoming examples include Bradley-Abregu next month, and Sylvester-Karmazin tomorrow. In both cases, the superior boxer and prohibitive favorite to win a decision would have to be considered to be in major trouble if the puncher is able to land something clean with authority (especially true in these cases - where Sylvester's punch resistance is exacerbated by the wars he's been in, and Bradley is the much physically smaller man and already showed a little vulnerability against men his own size).
I think it is a fallacy in the loose sense: the reasoning is incorrect. The reason would go like this: "X has big punch and thus if he lands on a weak chin he definitely wins". Nope. Does not work like that. Amir Khan can take a flush shot from Prescott if it lands on the correct spot (or incorrect spot, depending on who's perspective we are dealing with).
To be really accurate it should be "if Y connects in the right spot, at the right time of the fight/round and can follow up on it, it's over." Alos sometimes the slugger wins a decision by intimidating the boxer into passivity/defensiveness.