There are two ways to hold your breath. The first, and what most people will do, is to seal off their airway at the back of the throat. If you ascend and your airway is closed at the throat, lung expansion injury is a definite danger.
But you can also hold your breath with your airway open and only seal at the mouth. This requires more diaphragm control because you're using that muscle to hold open your lungs. It's like the isometric version of breathing. If you ascend while holding your breath with your airway open, air will just be pushed out of your lungs and through the lips, since that seal isn't nearly as strong. This is why SCUBA divers are taught not to hold their breaths and to blow small bubbles when their respirators come out because both practices ensures an open airway. But those aren't the only ways to keep the airway open.
For SEALs, I would imagine it's important to learn to safely hold one's breath underwater. Exhaling bubbles make you visible to both other divers and those at the surface and there are times that a SEAL team would need stealth underwater. Some minimal risk of lung expansion is acceptable when trying to avoid getting killed in some other way. I had a diving instructor who had originally been trained by the Jordanian military's SEAL equivalent who would breathe roughly once per minute. He said it was second nature based on his training but that we should learn to breathe the way PADI recommends since no one would be shooting at us.