When Necrozma blocked Mewtwo's attack and defeated Yveltal it was because he was enjoying the unlimited energy of Victini (see dex). This is also thanks to the same process that Mewtwo has done away with the moon, so for this scene it makes sense.
After Necrozma leaves the planet, and they all die by yveltal.
For the rest it is relevant because in Ultra form, it is the fighter with the biggest statistical base and that the light is not affected by the time because of its speed in addition to being diffused in all directions. It is therefore logical for him to take the upper hand over Dialga and Palkia (it is from this very concept that the logic of ultra-wormholes results).
On the other hand, the timeline Necrozma win is a little bullshit, I understand that Necrozma absorbs the light except that Arceus is not of the same nature as Solgaleo and Lunala so there is not very relevant either.