if you got more value out of the ER effect, then that would make up for that gap, no?

But you don't because he's overcompensating on dehya's energy needs by trying to force her into a different role and there's a lot of damage lost by trying to funnel particles into her. You get more damage out of adding characters that do higher personal damage (eg ganyu, wanderer) + reactions (eg nahida, kazuha, kuki) than you do by trying to "maximize" dehya's personal damage with bursting every rotation. Shooting for 220-240 ER also requires sacrificing a lot of substats.

Also, he is someone who rolled on the banner and got Dehya and is trying to make her work on his account

I am someone who rolled for dehya and has her c0r1 and I have been using her successfully and I find her to be very strong in reaction oriented teams. I don't need to worry about bringing someone to buff her anemic damage or characters to funnel particles to her. It's a good team that does quadratic damage off of burning, and aoe off of swirl, and overload.