We don't know what happened to Jay's power. He might just not be able to connect to his Speed Force here. And I don't think stealing speed is part of Hunter Zolomon's powerset. That would actually be Cobalt Blue.
Speed Demon would be the Black Flash who is just the manifestation of Death for Speedsters. Would be weird. Also I don't trust anything show creators say since they will obviously not want to spoil anything and use red herrings instead.