This statement is believable "Boost Threshold And Turbo Lag Aren't The Same Thing"
This statement is not "Turbo lag is the additional time delay above boost threshold, when the engine’s throttle valve is open and the turbocharger needs to accelerate to deliver positive pressure to the engine."
Think about this statement "Basically, boost threshold is the engine speed at which you have enough exhaust gas flow in order to create boost."
If the engine is at the boost threshold, the point where it can create boost because there's enough exhaust gas flowing through the turbine, then there is no lag, because the turbo is already in a condition where it can produce boost.
Plus the definition of turbo lag is inadequate, it cannot be measured. It sounds like it is supposed to start at boost threshold (time delay), but when does it end?