I'm a bit mystified as to what he told me over the phone that they are
positioning the thermostat in the hallway.

Can somebody enlighten me as to why the hallway of all places?

It is traditional because when central heating was originally installed,
people often still had additional heating in the lounge. You can't put the
thermostat in the same room as an additional heater (i.e. gas/solid fire) as
it will fool the system into making the rest of the house cold.

However, Luddite plumbers still put the stat in the hallway even though
people haven't installed ugly gas fires since the 1970s.

The room that the stat in has the most comfortable and reliable heating, so
you want it to be the most frequently used room. You need to balance it and
choose the radiators such that it is the slowest room in the zone to heat up
so that the other rooms' TRVs close off whilst the boiler is still firing.

Christian.