I've rewritten this several times to try and make it clearer but it seems to be getting less clear with every pass. Hopefully getting to the nub of it:
Chimeric totems like a 'true self' seem to me sort of sacred brainfarts. There's a religious intensity to the idea - a pursuit of an impossible goal, something that is always beyond the here-and-now. People hooked on the promise of something better than bog standard here-and-now reality.
Claiming the self is a fiction/construct (which one could say has characterised so much thinking over the past hundred odd years - from Darwin, Freud, post modernism - we could even go back to Galileo!) seems to lead some to suggest that there must therefore be another 'self' that is 'true', hidden underneath the 'false' self. (A false dichotomy).
The 'true self' doctrine seems to reject the suggestion that self is constructed (by society/history/narrative/culture/context) and cling to the idea that a self is solid, true, immortal.
I guess at base it's just seeking to resurrect a solid, true, permanent idea of 'self', because the idea of a 'self' being transient is difficult to accept?
Similarly, 'Genderism' posits that 'gender' as a social construct is a fiction that can be deconstructed/swept aside, and simultaneously claims the existence of a 'true' 'perfect' 'sacred' gender, known only to the individual.
Humans really struggle with uncertainty and are very attached to the idea of a self and a soul. We find it very hard to accept that so much of life is unknown, negotiable, shifting, transient, predicated on delicate and interconnected networks.
At root it's a plain old escape from mortality, I suppose?