[gindely]

You're radically misunderstanding that definition.

It has to do with the notions that are expressed by "can, should, might, must, has to" etc. These words (most of them are called "modal verbs" in English) modify a proposition in such a way that they do not mean it happened/happens/whatever, but qualify it, or indicate its possibility etc. So "he's going out now" is a proposition. "He can go out now" modifies that proposition - no longer are we affirming it, but merely affirming the possibility or permissability of it.

Nothing about "are you sure" is modal. A modal dialog is called modal because the mode of the program has changed - no longer can it accept requests to delete articles or add text or modify an avatar, but it is now in a mode when you can either say "delete this" or "don't delete this".

In particular, if you showed an inline prompt that says "are you sure? delete/cancel" but still allows you to interact with the rest of the system, it's not modal.