You can read the whole thing (it's pretty good), but there are several points to be had in the piece.
-There was booze at the party but people brought it with them-- it wasn't served. From the accounts, the party actually sounded kind of lame. The people who later assaulted the choral group were uninvited.
-The fight did start over a confrontation over beer, mainly that Aziz tried to grab a beer the uninvited guests brought with them, but it kicked into high gear after the group sang the national anthem. That brought the now famous quote: "this is the gayest s -- I ever heard. What a bunch of fags."
-Afterwards, Aziz and the rest of the group went outside to talk to the uninvited guests to try and calm things down. The choral group pretty specifically said they didn't want to fight and mentioned that they outnumbered the others, mainly to prove how fruitless starting something would be. To which they heard back: "this is the 415" and that this was their (the uninvited guests) territory. SFist thinks anyone who says "this is the 415" deserves whatever they get.
-The fight was your basic group assault-- somebody was hit and if they fell down they would immediately get pummeled while on the ground. Nice.
-Not only did members of the choral group stick around afterwards to detail what happened, they filed reports the next day. This is the part that has been denied by the police department and this is the part that sounds pretty much dead wrong.
So, to sum up-- bad guys very bad, choral group pretty much assaulted, cops lying.