In other words, what I wrote above, they substitute a long loosely fitted leak path for a short tightly fitted one.
I have serious questions whether static leak tightness of the type referenced is actually what is needed in a musical instrument. Why do we care if the tenon "leaks"? Because it'll create a vent in the wrong place and try to cut off the standing wave in the bore of the horn. OK, what kind of vent is needed to be able to cut off the standing wave? What we need in order to have a problem, is a vent that can pass a certain (small) amount of air back and forth at some (tiny) level of fluctuating differential pressure between the peak internal pressure of the standing wave and the average pressure outside. I would suggest that a long circuitous leak path, while it might leak more in a static test like using a Magnehelic manometer, might well be better than the standard joint at preventing partial cutoff of the standing wave in the bore.
I am not convinced till someone brings data or calculations, that the effectiveness of "sealing" at pads or joints in the application of a saxophone, where "sealing" needs to prevent that in and out flow of air at very tiny pressure differentials and rapid alternation, is appropriately measured by applying a static flow test at a relatively high differential pressure.
It may well be that a long, loosely fitted leak path is actually BETTER for preventing the development of an unwanted vent, than a short tightly fitted one, given the inevitable inaccuracy of manufacture and wear of both types.
To understand why any leak is a problem, it is necessary to understand the physics. A standing wave inside the horn produces two different types of antinodes, whose position depends on the length of the air column. There are pressure antinodes and displacement antinodes. A pressure antinode is a place where the air molecules do not move, but the pressure varies. The displacement (or velocity) antinode is where the pressure does not vary at all, but the air molecules move. In between the antinodes, there are combinations of variations in pressure and in air displacement.
The end of the horn, or the first open tone hole is clearly a displacement antinode, since there is nothing to contain the air, so the energy wave displaces the air. The mpc tip is a pressure node, where air compression happens. Inside the horn there are complex things going on, as there are many potentials for where the wave will form, depending on the length (obviously) but also the shape of the bore. These are called impedances. Impedances also have maxima and minima, and just FYI in a sax the waves form around the maxima (the opposite for the flute). Anyway, my point is that at the pressure antinodes, all the energy that produces the sound at the end of the air column goes into pressure variations, and they are not insignificant.
At the end of the day, the question is really whether at maximum pressure at a pressure antinode, there is a transfer of energy out of the wave caused by the leak. It doesn't matter if the pressure is static as with a Magnehelic or whether it is caused by a varying standing wave, if at a certain applied pressure some of it is lost due a leak, then there will be consequences.
It's not quite like a garden hose, that takes time, once you turn on the faucet, to squirt out the end. That is down to two factors: the hose is not full and the hose expands under pressure. While it is true that air is compressible, the amount of air in any possible length leak channel in a sax is minimal. And even if the leak channel were long enough that a momentary pressure variation did not cause an "escape of air" to the outside, the pressure of the standing wave at the pressure antinode is still lost to friction as the air is compressed in the leak channel.
The only condition that applies in terms of using a static leak test is that the pressure generated by the Magnehelic be equivalent to that of the maximum pressure generated at the point of the leak by the standing wave. And generally speaking a higher pressure generates a leak by compressing non-rigid material. This is mostly to do with pads, where a finger or a spring is compressing a pad enough to make a seal able to withstand the maximum pressure exerted on it by the air column. As the pressure increases, it eventually exerts enough pressure to deform the pad enough to break the seal. This is obviously not the case at the tenon joint.