Pay and promotion is about making yourself valuable. If you mentor engineers and gain a reputation for doing so, you're effectively shaping the culture of the organization, not to mention gives you a ton of leverage with the productive employees you taught to be productive.
It makes you far less disposable. When mixed with even amateur negotiating skills, that leads to promotion and better pay.
But the 'den mother' comment comes out of a real issue, of womens roles being sidelined. One pragmatic response is to avoid roles that feed into the stereotype.
In a better world, this would not be a thing. But here we are.
You make a good point, and I agree with the pragmatism of the choice, but I also believe mentorship can be a means of advancement.
The point of my original comment was to advocate for another choice - one that will both advance the mentor and their proteges. Hopefully, this will give us that better world we're looking for.