Along I-70 in Kansas, the superchargers are often very, very empty. It's unusual to see a 2nd car. If I go to a restaurant and come out 20 minutes late, will Tesla still charge me idle fees?
Ah, well, there is the theoretical "answer", and then there is the practical "how it plays out in real life" answer.
According to Tesla's policies, if it is less than half full, there should be no idle fees. If it is at least half full, then yes, there would be idle fees. This is sensible and should work pretty well.
However...Tesla's detection systems have been awful at some times in the past about detecting the number of cars accurately. There is this 5 page thread from 2018 of people getting charged idle fees when they were the only car there, or when they were far less than half full, but Tesla's systems were inaccurately showing that the site was mostly full and was assessing idle fees. Hopefully, Tesla has fixed these communication issues some in the last few years, so the in-use stall count is more accurate now.
Idle fee at empty Supercharger?