No argument there. But we're talking about the upper limit of a closed MP vs. an open one. Blowing as hard as you can will ultimately cause the reed to close and stay closed. This happens at a lower pressure on a closed MP than an open one. So at some point the SPL on the closed MP goes to zero since the airflow goes to zero. Blowing just as hard on the open MP will not cause the reed to close up, producing a higher SPL at the higher flow rate which was unobtainable on the closed MP.Other things being equal, SPL is a function of root-mean-square sound pressure.
RMS sound pressure is in turn a function of frequency and the rate of airflow being put though the horn.
So I predict that for a single horn, two totally different players on two totally different mouthpieces will generate the same SPL, as long as they're exciting the same frequencies at the same airflow rate.
Since you know your physics, you know you can get more flow through a garden hose than through a straw. Same principle.
My point is, since the upper limit on flow rate is higher on a bigger opening, the upper limit on volume is as well. But I agree that there are many other factors at play (reed hardness, baffle, embouchure, etc.), and that an opening can be so big you reach a point of diminishing returns.