From my understanding, only on-weapon IAS counts. The first two attacks of whirlwind sequence always occurs on frames 4 and 8 after you start whirling.
It would be awesome if whirlwind could activate “on hit” abilities, but this would be a major buff and would have to be balanced accordingly. Lastly whirlwind must be UNTARGETABLE.
You should check out DHA’s post regarding bugs surrounding whirlwind, good write-up…