Benevolent here refers to the intent, not the outcome.
If I say "women are so caring" and then use that as a justification for why I shouldn't hire them, the result is a sexist policy with a benevolent origin.
You see this all the time with stereotypes - queer men are so good at fashion, Asians are so good at math, Black folks are such good athletes, etc. We might think these are compliments, but when they are used to deny opportunities they can be as harmful as any other stereotype.
Any sexist or racist prejudice will either be against one group (women are so weak), leading to a disadvantage for them; or benefit one group (Asians are so good at math), leading to a disadvantage for everyone else. Both are leading to discrimination based on gender or race. Calling one of the two benevolent is just a bad name.
The whole point is that the outcome is not benevolent.
But it's useful to have a nominative difference between sexism that has roots in hate ("women are hysterical") and sexism that has roots in a "positive" stereotype ("women are nurturing").
Those are two different types of sexism with two different root causes and need to different solutions.
That is indeed right in the definition, which you should read:
Hostile sexism reflects overtly negative evaluations and stereotypes about a gender (e.g., the ideas that women are incompetent and inferior to men). Benevolent sexism represents evaluations of gender that may appear subjectively positive (subjective to the person who is evaluating) but are actually damaging to people and gender equality more broadly