Alice had no idea what Uranus was thinking,she only knew her disciples were being severely beaten by him. As their master, she felt obligated to stand with them.
She was aware of her lower strength level but couldn't just stand by idly.
However, her joining the fight only made things more difficult for Uranus.
Initially, dealing with Loki and the others was challenging enough for Uranus, as he had to adjust his strength constantly to avoid killing them but still needed to strike hard enough to be effective. He knew that as long as he didn't kill them, William the Obliterator might not turn against him outright, at least not without a good excuse.
But harming Alice, even slightly, could spell real trouble for him.
Now, with Alice, a Destroyer Deity-level fighter, joining the fray, Uranus was thoroughly perplexed.
To avoid injuring Alice, did he really need to suppress his power to the level of a Destroyer Deity?