I get that part, but that brings up several questions. Why did it have trouble just idling? Most tuners tune in a single gear, they aren't tuning across a shift point so during a pull the BPV/BOV shouldn't be dumping air so they wouldn't even see that effect. Many manufacturers sell a BOV that can be made into a BPV by adding a recirc tube. Mechanically all BPV/BOV are a piston with a spring, a boost reference port, an inlet and an outlet.
The only thing I can think of is that the spring isn't strong enough under vacuum to remain closed and it requires some amount of vacuum from the recirc hose to pull the piston down in order to maintain a seal. Seems a bit unlikely. Either way, it means they had a pretty bad air leak somewhere if it required speed density to fix. Perhaps under boost it didn't matter and speed density was required only to allow it to idle as under boost it would be held shut by pressure on the boost reference port.