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Well, it does take an augment, but it's an invisible one The iota becomes a long iota but since they are both written the same way we can't see it. Check Smyth's grammar (or any other you want to obviously) about vowels and temporal augments (here's a link for Smyth's online)
It does seem a bit confusing, but fortunately Mounce is right and the form of verbs like ἰσχύω are very regular, so the 'σα' tense formative makes the aorist form easy to spot. There is a similar 'recognition' issue with verbs that take vocalic reduplication, where at first it can be hard to tell that a perfect has reduplicated and it almost looks like a simple augment. In both cases the important thing is to focus on the differences, and recognition will come.