Well, among your examples the lasgun one is just stupid, the guard's lifespan could be at best used in relation to one book/some war or sector, and still minutes = hours. The Alpharius one is a not funny joke about the difficulty of pinpointing who is who among the twins and in general the true goals of the Alpha Legion (more canon and probably funnier would be an argument about "I am loyal because I wants to destroy the Imperium" No I am loyal because I wants the Chaos feed on mankind and swallow the Imperium"...but people are lazy).
Fulgrim gay and DA are heretics are technically half-truths turned in half-jokes. The subtext in the Fulgrim character (and renditions) is there, we could also accept it as canon but the problem here is it's becoming a bit redundant (and not funny anymore, if it ever was funny in the beginning). About the DA, again people are too lazy to be accurate, and anyway saying "well, actually just half of them are heretics, but not even DA know who is who" is not funny - and it would lead to other Alpharius anwsers.
So basically a lot of us use catchphrases because it's too long to explain to the newcomers the truth (admitting we know it in the first place...), too boring being precise with people who already knows it and not funny being accurate when we just want to land a pun.
Personally I instinctively regard every one-line comment as an intended joke (and I am usually vindicated AND STILL ALIVE!) and every text-wall as an in-depth analysis, which could - or could not - be accurate as a Robot Guile Man battleplan and interesting as his own sexual life