What the hell even is that? You're either born ugly or you're not. It's an ignorant attempt at associating men's retribution towards women's bullshit as "basement-dwelling."
An involuntary celibate (INCEL) gets no pussy, because he's ugly/physically undesirable. Naturally, he might gravitate towards blackpill ideology or remain bluepilled and unaware of female nature.
Blackpill basically teaches them that women are and have always been materialistic and selfish, choosing the best men to breed with. You don't need to be an incel to know that.
Which is totally fine. It shouldn't be any different even.
The problem is that due to various factors, women indulge in hedonism, and so are the men they engage with. It's much more difficult for those who want actual relationship and family, not just hollow, transient pleasure via sex.
The men labeled mediocre aren't bad even. They may be fine husbands and fathers, possibly even great at that. But women do not pursue values that make men good husbands and fathers, but instead pursue factors like physical attraction and income. They overvalue their own physical attractiveness and indulge in being lazy fuck mattresses, and believe that's the height of the value they bring into a relationship.
Well, not all women of course, but there is a significant shift.