2.0 TDCI Thermostat Spiking On Hills

I have been having issues for weeks on my 2017 Mondeo 2.0 TDCI, where if I am on the highway and hit and incline, or on a country road doing over 50km/h and hit a steep incline, the thermostat begins to spike, and then if I let off the gas, it very quickly drops back down to 80 or 90 C.  Once I got an engine overheat warning at the top of a steep hill and it went into limp/stall mode for a few seconds, but then the temp dropped back down to 80 or 90 C again right away.  If I am driving locally or on the highway and not on an incline, the car drives fine.

The mechanic said I have issues with my water pump and my thermostat housing.  Does this seem correct?  My coolant level is fine, it seems odd that this issue only seems to happen on inclines, and I wonder how they came to the conclusion that they have to replace both these units.