In a house fire, how hot does the foundation have to get before it too has to be replaced?
The information is incorrect, the temperature at which concrete becomes unsafe for re-use is significantly lower, 570 degrees F actually. The 'telltale" sign is if concrete that was not charred from nearby combustibles turns a pinkish hue. That color change is due to chemical changes in the iron-containing compounds in the aggregates used in making concrete reacting with the Portland cement that is binding them together. Once it turns pink, it is unusable. The fracture mechanics are permanently changed and it will crumble under stress. That has nothing to do with the rebar inside.

Even IF it is not noticeable damaged, you may not be able to re-use it, or want to. After a fire where the rest of the house is completely destroyed or had to be demolished, when you go to try to re-use a foundation, the Building Permit code requirements will be "reset" to the current codes, you are not longer "grandfathered" into the code requirements from when it was originally built. But since it already exists, the inspectors cannot see HOW it was made. So they will require an Engineering Report from a licensed Civil PE certifying that the foundation meets or exceeds the current building Code requirements. To get that report from anyone other than a scam artist, the PE will require core testing, wherein a company comes out and drills cores from your foundation and takes them to a testing lab to test the strength. In addition, they may require a survey / map of the foundation for rebar locations (done with a metal detector). By the time you get done with all of that, it can be so expensive that tearing out the old foundation and replacing it will be less expensive.