I think that there is a crucial difference between an incel thinking these things and being subjected to insecurities about how they may present themselves and women who end up thinking these things.
In my opinion, incels are more prone to insecurities based off of real experiences they might have in their lives, feelings of unfulfillment and trying to rationalize exactly why they are incels, and with that way of thinking, they display tendencies of becoming insecure about a variety of different things, these thoughts take place after incels are sheltered for a long time with idle minds and they want a reason as to why they are incels. Meanwhile, these women you were referring to, were perfectly normal women with no real reasons to be insecure about many different things. Such as the example you mentioned, with the nails, it holds no real meaning but they still think these thoughts even as perfectly normal individuals with no real reasons to be insecure about these things. For them, it's a form of bonding experience in which they display insecurities to others and expect a response in which another person is able to identify themselves with these feelings of not being good enough, and they do this despite having perfectly normal lives with no real reason to be insecure.
On one end, you have people who live lives that aren't optimal, and based off of that, they find reasons as to why they live the lives that they do, which they do not consider to be very good. On the other end, you have women that have lives which can be considered worthy by just about anyone, but express insecurities as a means of either bonding with other women who are also insecure, or as a way of passing their time in a way that emphasizes some emotional qualities that females are more likely to show.
For a man to display insecurities and emotions, it is something uncommon, so if you are an unsuccessful male, then you feel like you have less reasons to live up to the reputation of being a man, as many do.
I'm not sure if it makes sense, but at least that's the way I see it.
I'd also like to point out that there are many incels who have been emotionally neglected by not only their peers, but even their own families. I would argue that due to this emotional neglect, they end up being more emotionally volatile than a normal person would have been, stuff like socializing and girlfriends are things that contribute to a state of mind which could be said to be stable, and neglect in many different forms means that they spend more time and effort among individuals such as other incels, to try and identify with a group that is open, they spill their emotions onto other people who they feel like are similar to them. So, with that said, I would like to point out that trying to rationalize the mind of a person through interaction of something like imageboards alone, is simplifying it a bit too much. Many incels have the desire to fit in, so they spend a period of time analyzing an imageboard, trying to shape themselves into something that is already present, and then interact based on that. There's the incel that talks to people on imageboards, the incel that is like many other incels, and then there's the incel outside of imageboards, which can be an entirely different person from the imageboard incel. We'd need to know an incel in his entirety rather than trying to make sense of imageboard incel, that is the only way to actually be able to properly rationalize incels.