From the tweet:
"Shared psychosis involves a dominant person or group imposing false beliefs on a submissive person or group. It shares features with the abusive relationship style now recognised as coercive control. At the start of a shared psychosis submissive partners are aware that other people don't share their dominant partner's delusional beliefs, but they lose this awareness as their own psychosis develops.
Over time, the dominant partner limits the submissive partner's opportunities to reality test by limiting their contact with other people. At some point, the submissive partner is so convinced of their partner's delusions that those beliefs become almost impossible to challenge. This leads to further isolation and dependence."
This is persuasive. But it does differ from conventional usage of the term in that shared psychosis is usually thought to be self limiting, i.e. one of the parties has a real, sever psychiatric condition like schizophrenia and transmits their delusion to someone within their sphere of influence due to that person's susceptibility. But the person who acquired the delusion does not acquire schizophrenia, and is typically not thought to be at risk of transmitting their own delusions to another without the added influence of the first party.
Andrew seems to be arguing, though he doesn't state it explicitly here, that there is enough diffused mental illness among the social networks of people contemplating transition, transitioning and having transitioned (including, presumably on line) that a severe form of psychotic mental illness in any specific individual is not needed to precipitate the delusion in susceptible people. And the fact that transition tends to attract people with severe mental illness (though mostly non-psychotic) and debility, that that maintains the potency of the diffused mental illness in the cohort, even though few have a psychotic illness per se.
It does seem speculative. But it's a serious attempt to link the socially contagious nature of the trans identity among young people with the extreme dis-regulation of the community as a whole, and high (but not uniform) prevalence of mental illness of those who seek to transition at a young age. Something that we've all observed.