Yeah, I can't think of a worse way to try and bring in a bioweapon than with an uncontrolled, or at best, semi-controlled balloon with a giant signature. If they were gonna do it by air, they would use UAVs with stealth tech and release at low altitude.
But aircraft are a dumb way to do it. Much easier to bring a bioweapon across the Southern border, release it on the ground in various crowded locations, and then make us figure out who brought it in and where.