Like much communication, there is something unstated but clearly implied in the expression "There is no such thing as a free meal" This is that it refers to products of commercial enterprises. Charities, for instance, do give meals away for free. Linux is produced by a volunteer, non-profit organization, and so it is free.

By the way, I have noticed that it is rather common for HN commenters to fail to pick up the assumed meanings, and jump on the words alone as being mistaken.