Philosophers and others distinguish different kinds of rationality – such as, theoretical rationality (rationality of one's beliefs) from practical rationality (rationality of one's actions). Instrumental rationality is the rationality of one's means given one's ends, which according to some philosophers (e.g. Humeans) exhausts practical rationality (putting one's choice of ends themselves beyond rational criticism), whereas others (e.g. Kantians, Thomists) believe that one's ends themselves can also be subject to rational judgement. Some philosophers want to reduce some of these types of rationality to others, something which other philosophers object to–on that topic see https://www.princeton.edu/~tkelly/erair.pdf
I don't really have an opinion on whether or to what extent Terry Davis was or wasn't rational. (I don't really feel qualified to have such an opinion, especially since I never knew him personally, and also since I lack knowledge and/or experience with his illness.) But it is interesting that people are throwing the word "rational" around without appearing to be familiar with the complexity of the debates about how to even define it.
TempleOS is an example of rational thoughts, assembled with intention.
Mental illness and rational thought should not be conflated.
If you're smart, but irrational, then the things you do will exhibit 'interestingness' but won't be connectable to existing bodies of knowledge and understanding, i.e. out of the norm. You have to work harder to understand what's going on, to connect the dots between the elements.
In everybody's brain, thoughts are connected with each other because brains are organized to provide continuity of experience. Rationality concerns how well you can convey that continuity to others.