Thermionic emission just refers to releasing electrons from the surface of a material by heating up that material (which is what happens in a vacuum tube). But

Thermionic emission just refers to releasing electrons from the surface of a material by heating up that material (which is what happens in a vacuum tube). But it doesn't specifically deal with what happens to the electrons after they leave the surface of the material. I am not sure what part of the wikipedia article you are referring to – the word "insulator" isn't used anywhere in the article.