Is psychosis classed as a disability?
Yes. Yes it is.
Psychosis is a severe mental disorder in which thought and emotions are so impaired that contact is lost with external reality.
Let's refer to the definition of disabilities given to us by WHO:
Disabilities is an umbrella term, covering impairments , activity limitations, and participation restrictions . An impairment is a problem in body function or structure; an activity limitation is a difficulty encountered by an individual in executing a task or action; while a participation restriction is a problem experienced by an individual in involvement in life situations. Thus, disability is a complex phenomenon, reflecting an interaction between features of a person’s body and features of the society in which he or she lives.
— World Health Organization, Disabilities
As you notice the word "impairments" which also means the state or fact of being impaired, especially in a specified faculty (either physical or mental impairment).
Also, note that it mentions "a problem in body function or structure". So when the mind does not seem to function well that it causes distress to the people around, it is considered an impairment.
The next sentences seem to support that fact.
Hope this helps! :-)