Pretty Sure theres a mistake here
https://pokemongo.gamepress.gg/sites/pokemongo/files/2018-11/Gengar%20Counters%20_%20Shadow%20BallGP.png
This shows Mewtwo with Psychic having more DPS than one with Shadow ball, which isnt true unless the weather's windy
Answers
So here's the thing. That chart is specifically showing a simulation of sucker punch/shadow ball Gengar - if you run that simulation on Pokébattler, it does show Shadow Ball slightly ahead of Psychic (only by 3 seconds)
The difference is much more prominent against other movesets.
However, these charts use GamePress's own battle simulator (GoBattleSim) which may have the numbers slightly different to where psychic is a tiny bit better.
In reality, the two are both good options to an opponent weak to ghost and psychic, and you will have a degree of RNG and variance with each raid you do anyway.
Given that every other Gengar moveset (sludge bomb and focus blast) have Shadow Ball with a much larger DPS difference, I would personally consider that one more valuable - but your Psychic Mewtwos are still a very good option here.
