I don't think it really came from anyone.
It was just repeated by both pro and anti Russian forces, because that's how long people expected it to last.
Putin allegedly said that he could take Kyiv in 2 weeks back in 2014, but that's another story.

I mean I don't particularly have a lot of sympathy for a sanctioned Russian propagandist who both denies genocide in Bucha and advocates genocide at the same time but you are right in that this is a lot more barbaric than usual for FSB assassinations.