IR sensors can only read surface temps. The most important temp in this kind of oven would be the cooking surface which is easily read with an IR sensor. The deck may be very hot, but may not be indicative of the temp of the entire thermal mass of the oven. If you're cooking one pie, this may not be much of a consideration, but if you're cooking many pies, the energy stored within the mass of the oven is important for maintaining the temp of the deck. My oven has a probe embedded in the brick of the dome. It is very useful, but not essential, in letting me know when my oven is ready. After many firings of the oven, I now have a pretty good feel for when it's ready, but it was nice to have while I was learning.
Another reason for some kind of probe in the oven is if you are using it for baking bread with the door closed so you can monitor the air temp without having to open the door which lets out heat and moisture.