If one is staying an an Airbnb that has an electric tea kettle, may one assume it is kosher to use, since normally people only use such things to heat up water and it is extremely unlikely any other food has gotten into it?

ב"ה Shalom As you said, it is extremely unlikely that is used for anything other than water, and therefore assumed kosher. If you want to take extra precaution, you can simply fill it once, boil it and then empty the contents and then it is fit as use normally. All the best