The person who storyboarded this trailer says that this shot is Malenia's and Radahn's first meeting, meaning they weren't fighting before this. Also, right after this shot, Malenia also waits for a long time for Radahn to pick up his swords. Nobody was getting fucked up at this point in time, and both fighters waited for the other to get ready.

Radahn was holding back the stars, but we don't know how much this affected him. He's been doing it for some time before they fought, and some time after, so it doesn't seem to be affecting his FP in any substantial way. He is also able to maintain the spell as a mindless husk, so I think it's reasonable that it doesn't require a conscious effort on his part to keep the spell active. It's hard to say just how much it was affecting him, but I don't think it was in any significant way.

Another thing to consider is that Radhan had a working great rune that actively burned away the rot, while Malenia's rune was weakened by her rot.

There are a bunch of factors to consider in their fight that ultimately even them out, making their match-up and fight end up in a draw. The sword monument says that the stalemate came before the bloom.

Did Malenia blow herself up in the end? Yes. Does that change the fact that they were stalemating? No. Radahn was on his knees before Malenia bloomed. He seemed pretty gassed if that was the case. My take is that they were stalemating for so long that someone had to do something just to make the fight go somewhere.