She is speaking to the daughter of a mountain, so it would make sense to tailor advice to her.
There is a race called mountains, who along with race called rivers, are both literally mountains and rivers and sapient beings. One clear example is when Ganga is described as both a literal river and able to talk (makes more sense if you imagine the rivers as flowing in a circle) in a way in which if they were not the same thing the story would not make sense.
"Lomasa said,
...
Then came down from the sky Ganga, the daughter of the snowy mountain. And her whirlpools were raging, and she was teeming with fishes and sharks.
O king! she directing her course towards the sea, separated herself, into three streams; and her water was bestrewn with piles of froth, which looked like so many rows of (white) ganders. And crooked and tortuous in the movement of her body, at places; and at others stumbling at it were; and covered with foam as with a robe: she went forward like a woman drunk. And elsewhere, by virtue of the roar of her waters, she uttered loud sounds.
Thus assuming very many different aspects, when she fell from the sky, and reached the surface of the earth, she said to Bhagiratha,
So presumably Parvati is a mountain or river given her family. Thus being absolutely gigantic, beatings wouldn't really hurt her and she would be fine. Thus beating your spouse for mountains and rivers makes more sense than for humans.