The Emperor mentioned it at the end of the episode.
I think "weakened" might be overstating it though. He did decisively batter Kenobi when Kenobi was back to his ROTS level. Even is he wasn't quite as good in Sabers.
Vader's hatred for Kenobi did leave him unbalanced and reckless (which is partly why he lost in both ROTS and OWK). Sidious noted said recklessness in that recent comic issue.
Vader basically went full-Maul and let his hate get the better of him against Kenobi
^ Agreed. Having said that I'm not sure if peak Vader at that point would have been > Amped Kenobi at the end of that series. Kenobi at least seemed > Sabers (even before his amp).
Personally I think awakened!OWK Kenobi would have just been above Vader regardless at that point. But Vader's hate-fueled recklessness certainly didn't help his cause.
Kenobi went god-mode and became (almost) as powerful as noob Rey. Create all the excuses you want, but Kenobi treated Vader like a toddler at the end. It was a complete stomp.
Probably not the best comparison tbh. Kanan didn't disarm or cut Maul, merely defended against his blows for 3 seconds before pushing him off a cliff ledge.
Also Kenobi's boost seemed more permanent, given how he dealt with Maul a few years later.