I would say that mansplaining is an actual real phenomenon, sometimes directly so (guys that do believe they know better than women) and other times indirectly, in cultures were men behave in a more assertive and aggressive way than women in conversations in groups and take over the conversation, in practice creating situations were they've taken over explaining something which a woman in the same group is vastly more qualified to explain.

However the word "mansplaining" has been so massivelly abused by mysandrists cosplaying as Feminists (to the point that in the last couple of years the most loud and frequent use of that word was by said mysandrists making prejudiced statements), that now there's a natural "bullshit!" reaction to anybody using it.