No worries! If curse is intended to work as you described, I think the language should change, at least. "Interrupt" in my mind means that it will be interrupting them at any point within their ult
Curse does not currently interrupt warden, infernus, or viscous ults.
Match ID where curse was used multiple times against warden and infernus (Vindicta and Paradox bought it towards the later stages of the game): 33992766