Firestorm spell, Incinerate spell and Elemental Discharge support gem interaction

To Critically Hit skill must apply its damage as Hit, not as Damage over Time as explained in tooltip about Hits. Firestorm can do Criticall Hits, it has Critical Hit Chance value listed in its tooltip and also has "Sustained" type\tag, which assumes a spell is doing "large number of Hits over a period of time". Therefore, I assume that Firestorm is a spell applying its damage as hits.

Incinerate, on other hand, cannot do critical hits, as is listed in its tooltip. Moreover, its tooltip provides info that this spell only applies Ignite ailment as though dealing damage (but it is not dealing this damage by hitting an enemy).

Now we have Elemental Disharge support gem, what supports spells what can Hit, as is explained in its description. It can be slotted into Incinerate spell, but not into Firestorm spell.

I guess, this is not intended behavior.