This is very complicated. Sure, a thermal bag is a bag, end of story, but what do you plan to put inside it and where?
In general, carrying cooked food across security is near impossible where liquid restrictions are in effect. You’d need to have some food which does not have any liquid and even then they might deny you because pasta, certain kind of sausages, cheese spread etc count as liquids.
It becomes more interesting if you buy food at the airport past security because some airports will allow you to buy food you can carry on and if you move a cooler bag from your carry on to the inside of the “food on the fly” paper bag just so the food lasts longer, I do not see it causing a problem.