Actually, Will would only say that if he were a liar, because Will didn't feel that way. Will was honest and direct; he didn't rant and rave, and neither did Matt. They both said their piece, and it was done. I'm not finding fault on either side of that exchange.
I don't think it's unreasonable to have different standards for siblings than for distant cousins.
Will and Gathan have argued about Zach, but I don't remember where they're not close anymore, or how Will has treated him like dirt. I think this is something they disagree on. Happens to friends from time to time.
Lets remember that it took Will much longer than a day or two for him to forgive Zach for sleeping with John. The incident with John happened in the Spring of 2000, and here it is, the Fall of 2001, when Will is finally, uh, friendly with Zach again. I think the counter-argument, that Will carried a grudge for way too long, is much more plausible.