Again, I think that's pretty extreme considering his excitement towards the coming fight. Ansbach as strong as he once was, and often downplays his current abilities. Yet he was still willing to partake in the fight with a balance of confidence and worry, and to say he dies instantly makes no sense when looking at the context of the entire fight. There's no reason for fromsoft to build this importance around them being a part of the battle just to die immediately. That's like saying Siegward did absolutely nothing in his fight against Yhorm and that the Ashen One did all the work in DS3.
That's what I'm saying though. I agree that Ansbach is bellow Mohg, that's why I'm arguing if Mohg should upscale from Ansbach. Hell, Mohg was on the path to become a lord and arguably would've become one if Miquella didn't want Radahn instead. Plus, at the point in the game where we face Mohg (and Malenia too) we are already on the path to fighting high-tier characters like Vyke and the Fire Giant. Hell, Malenia herself can be argued to deserve high tier scaling with her status as a Goddess in her fight. It's honestly fitting too that the two bosses of the two secret areas in the base game are a would-be lord and goddess respectively.
Overall, I think Mohg could at least get a "possibly 4-A" rating, instead of the "likely 4-A" rating that all other lord level characters have to differentiate if you want.