Whether or not the balloon popped isn't really salient to self defense issues unless your life depends on popping, not just deflating, balloons. For defensive isssues, people are not made of latex and overpressured with air on their insides.
What is not described in the query is where the balloons were being struck by the incoming rounds, if the balloons were filled to the same pressures, and if the balloons used were all of the same size, shape, and thickness of latex. In other words, depending on the balloon and inflation, results may vary.
There are other issues as well. In particular, how were the balloons mounted to be shot? Sometime back, we did a "Women's Revenge" sort of shoot where we used slightly inflated balloons to represent the scrotum in an IPDA silhouette target. We were surprised to find a huge inability of the shooters to rupture the "scrotum" even though holes in the target behind the balloon indicated that the balloon should have been hit. What we found was that the under inflated balloons were pushed out of the way, in part, by the pressure change created by the incoming rounds and that the lack of inflation gave the balloons a lot of flexibility to handle the brief insults.
Here is an explanation for popping balloons, or not, I borrowed. The key words are "Tensile cohesion."
This is from the 2/9/96 Charlotte Observer. Each Friday, they run a "You Can U" with a Beakman and Jax segment in the comics section. It deals with an explanation of a child's science- related question of the week. This weeks question: "Why do balloons pop and why do they make a loud sound" (sent in by a child with the appropriate last name of Huff).
The response deals with how a balloon breaks. The explanation: "Here are two big words: tensile cohesion (TEN-sill co-HE-shun). They mean *stretchy strength*. Balloons break when they get expanded beyond the limits of their stretchy strength."
Then they give the kids two experiments (demonstrations) to do athome: blowing up a paper bag until it pops (to illustrate air compression and the related explosion), and what appears to be the needle through the balloon gag. Here's their run through:
"You Need: Balloon. Bamboo Skewer. Blow up the balloon and tie its end with a knot. Find the spot at the top of the balloon that is darker in color. Gently twist the skewer into the very top and out near the knot at the bottom. If you're careful, the balloon will not pop."
"The Balloon is made from latex, a substance that's built like long stretchy springs. When these springs get stretched too long, they snap and the balloon pops. The top and bottom of the balloon are like springs that are relaxed and are not pulling away. Everywhere else on the balloon, these *springs* are stretched tight near the limit of their strength-near the limits of their tensile cohesion."