I am aware of how the damage formula works, which is why I mentioned before that the difference is marginal as long as you can add any amount of attack up. The position you are taking is a nothingburger argument. You are exaggerating the totals when the formula is fractional to that. Its 1.2x1.2x1.2=1.728 compared to 1x1.2x1.3=1.56. Its marginal. And again will become more so as you add easy to find attack up.
Attack up is flat out easier to find than NP damage up. It is flat out easy to add to any team setup. There is no tier of buffs, just how common they are to add to your NP damage formula since NPs are king. The bonuses you get to face cards are insignificant in a properly set up team, much less an arts one that will be using NPs either each turn or every other turn for an extended period of time. Tamamo's high skill uptime meaning her setups will do a high level of work over time since they almost never go away.
Skadi offers 3 turns of damage uptime and then 3 turns with only her NP crit damage buff potentially available. Merlin will offer 3 turns of attack and buster, with 2 and 3 turns of downtime to each buff. Tamamo will offer 3 turns of buffs, 2 turns of downtime, and if you get her NP in that period only a single turn of downtime. Double Tamamo if that is your choice will potentially offer 6 turns straight of her buffs. And her total damage potential over that time is overwhelmingly powerful, and is basically the only easy setup to do 4 or more NPs in a row.
Tamamo's main weakness is a lack of battery, so most of her teams will use kscope removing the ease with which BG can be selected. Having NP damage up on for the majority of cards is absolutely critical considering that fact.
As for Chen Gong, I've seen him used to farm steel, which has the 180k valkyrie at the end to clear. The easiest video to find is with the new buffs in jp, but the equivalent aren't particularly onerous to find for cheaper considering Caster Gilgamesh's total buff output being much higher than Sherlocks even with his latest buff.