Yeah, that's pretty likely. I think it's also important to remember the impact of the quarry. The quarry explains how Alexandria has survived this long. Very few walkers managed to get through to plague it, so the walls were kept safe. But the quarry also kept the walkers away from everyone else in the region, which probably includes the Wolves. So, it's not just the Alexandria people who think they're tough badasses. It's everyone in the area, and all because they've never had to contend with a real horde or any serious buildup. So, the Wolves, too, think they're really tough, and probably figure they'll lure maybe a few dozen walkers at most into Alexandria.
Another option is that the Wolves attacked Alexandria when they saw the armed group leave the compound, and then tried to, like, drive a Mack Truck up to the gate to bash it down. Whoever was on sniper duty managed to shoot the driver...who then slumped over and died leaning on the horn. With the town under attack and the sniper firing away, there was no time to get out and clear the body off the horn...and now the walkers are coming. So it's just crap timing all around and everyone loses.
That's the best idea I've heard so far about why the horn is blaring.