You're right that is how it generally operates, but in the case of Comcast I think this meme doesn't want to die because in the late 00s Comcast really did do DPI to interfere with torrents: https://www.techdirt.com/articles/20071029/020756.shtml

Fairly googleable with "Comcast sandvine". Afaik they haven't done anything like that for years, though.