Okay, was Rhaast Vladimir's boss or not?

On one hand, Vlad's bio says an old Shuriman account "speculates that Vladimir’s master was not imprisoned like so many of his kin, but instead died at the hands of his own warhost." and that Vlad "struck the killing blow." In Vlad's color story, he said he killed his master. Someone who survived the Darkin War should know the difference between killing a Darkin and sealing one, and if he's going to kill Maura when she's done with his portrait then there's no point lying to her.

On the other hand, Vlad's portrait had him holding a banner with a SCYTHE on it.

So where are we on this? I mean I'd be thrilled if it WAS Rhaast because it means we got a concrete Rhaast backstory, but then why does the lore leave it relatively open-ended?