In the US, being called an Uncle Tom is to be called a traitor to the group you were born into. For example, it might be used to shame women who might sympathize with men’s plight, etc. The primary idea is that what group you were born into should for every member totally define their world view. If this is not sexist, racist, etc. then probably nothing is.
It is used as a method of enforcing solidarity with the group; and furthermore implying that if one leaves the group, then one might not receive any protection from the group if times get tough. This coercion is typically needed due to the fact that these groups often have world views that make little sense and worst of all, counterproductive to the individual in their day to day living.
For example, Feminists with their implied and real hatred for men make it difficult for women to marry men. Men see the irrational hatred, and it is always in the back of their mind when they deal with women. “Is she a crazy feminist who is going to use some canard to justify her bad behavior” men are always thinking.
Furthermore, just having a poor model of how the world works and what is happening in it does not make for effectively navigating it. This should be obvious, but apparently it is not.
So when Madeline Albright and Gloria Steinem try to keep women on the Hillary Reservation by effectively calling them Uncle Toms we see the dynamic in action. Hillary is not a particularly appealing candidate, so it is unclear how it is working in this specific case. Still we see the dynamic in action, and one can question why this is not considered to the epitome of sexism.