His height wasn't the main reason he went undrafted he went to a college that most people didn't know existed. He was literally under the radar except for the Saints.
Your right, im sure that wss probably a big factor as well, but not the biggest factor.
In the NFL, if you have talent they will find you. I mean just take a look at Adam Trautman. There has only been one other player make it in the NFL from Dayton. Trautman was drafted in the 3rd round. There are hundreds of small school guys that have been drafted. Unfortunately there are still some specific measurables that can scare some GM's away. Especially when you look at the history and it shows that people of his size have rarely had success at the receiver position.
Some people get real caught up in that, and sometimes lose out on a gamechanging player.