According to the article, it's not that simple. This is from the ToS for the Unity Editor, which is subservient to a broader Unity ToS that has much stricter legal language about changing anything without warning and the customer being able to go fuck themselves.
So, yes, technically this bullshit may be completely legal. Devs who were sold Unity on "no royalties" may be forced to pay royalties. Which is definitely healthy for our society and not obviously a problem.