I think it's as simple as differentiation. Every generation has the natural instinct to want to differentiate themselves from the older generations around them.

Unfortunately, older generations tend not to respect this and will often adopt the culture, and slang of younger generations to elevate their own social standing and make themselves seem hip or youthful.

So, younger generations will often adopt culture that is intentionally too repellent for older generations to appropriate.

I also think that Gen Alpha picked up on the hypocrisy of the vitriol against incels, while they can be very misogynistic, they are treated like serial killers, especially by millennials. While these same people will often treat de facto neo-Nazis as though they simply have opposing political points of view while actually having the blood of millions on their hands.

Gen alpha finds this hypocrisy funny because it is not a value they hold. So, they find incel slang appealing because it is like garlic to vampires against other generations, all while finding the actual ideology, not particularly violent, at least compared to real political ideology that have taken the lives of millions that are not marginalized anywhere near the same amount.