No- we aren't ripping anyone off. You're not getting where the joke is from.
Angry Marines is a joke that they came up with on a forum called 4-chan. It's a bunch (like, a ton) of different people who all helped come up with it over time, no one in particular. Many, many different people contributed funny artwork to the idea to keep the joke running. It's just an internet joke among 40k players. The codex is a total joke in the same way if you read it, and is just tongue-in-cheek with some of the artwork compiled into one source. The facebook page for them is the same thing- it's for people who are in on the gag from 4chan and like the artwork. They have many other running gags, like the vindicare/farseer love story or Kharnn wearing a commisar's hat.
Our league name is basically the 40k equivalent of a pertinent pop-culture reference.