First poster could have approach better too. Like "Cool site! I think I may see an error on one item?". Instead of going right to a 'wrong' angle as if all the data should be discredited. I get highly triggered by this too.

this overly positive attitude triggers a bunch of people too. wrong data should just be called out, especially if that's your main selling point.

Nobody disagrees with that. But tact exists.

But why is condescension tolerable when the person is right?

It is not but its order of magnitudes worse if the person is wrong.