Exactly. Pace Aegeri, this is not "extremely debatable". As he himself said, it's only an interrupt if it has to be in order to function. Variable resistance still does something, and it happens to be something fairly interesting IMO, if it's not an interrupt, so it isn't. The standard isn't that it's an interrupt if it's more powerful as one. Nearly anything would be more powerful as an interrupt than a reaction. It's that it's an interrupt if (and only if) it needs to be in order to work at all, barring cases where the ability specifically says something different.