I take it (in lieu of some real reading on the subject) that a
programmable thermostat relies totally on temp settings, just
that these are controlled over time such that off is actually
some sensible frost protection.

Indeed. It is one of the many advantages of having a programmable stat over
a programmer and separate fixed output stat. Another is that the override
controls are in the lounge, rather than a dark and manky understairs
cupboard. There are many more still, such as suitability for multi-zone
systems.

Christian.