CRM is not only about how one member of a flight deck crew deals with the other(s) - far from it.
In a single-pilot crew, your dealings with Ops, Refuellers, caterers, airport security staff, bus drivers, handling agents etc. etc. are all subject to the principles of CRM.
And when it all goes pear-shaped, the brown stuff contatcs the air conditioning and the flight deck is lighting up like a Christmas tree, you may well need to use CRM in your dealings on the radio with ATC, engineering, fire services, S&R, etc.
So single-pilot CRM is very far from being only a multi-crew discipline.