They're following the same unit research path as Dvar/Kir'ko: T2 Flier first, then three T3 units. So the dragon is more likely to be one of those T3s alongside the artillery and Tactician. Most summons are T2 or T1 anyway.
Guys, watch the stream. There is no speculation here, Firebrand is T3, you can clearly see him after Propagator in Shakarn military research tree...
They don't show the whole tree mind you but I think the full branch goes like this: Propagator > Firebrand > Refractor > Tactician > Sonokarn