The is still a pressure zone there, it is just not effective to as high a frequency.
The depth of the pressure zone is determined by the frequency being measured. At low frequencies the wavelenghts are large so the boundarly layer or pressure zone is deep. The same acoustic principle relates to 'Room Modes' which are active when wavelengths correspond to room dimentions and why speakers sound bassier when placed against the wall or in the corner.
At very high frequencies the wavelengths are short and the boundary layer or pressure zone is thin. The speaker acoustic analogy in this case is the 'baffle-step' in the speaker's frequency respose determined by the cabinet dimentions, and explains why soffit mounted monitors must be flush mounted without any gap between the cabinet and the wall surface. The same principle explains why omni mics become directional at high frequencies and at what frequency they will do so in relation to their diameter.
To effectively harness the boundary / pressure zone effect in a microphone at the highest audible frequencies the capsule must be very close to the surface. PZMs do that by mounting the capsule a tiny fraction of an inch above the surface facing toward it. The other omnidirectional boundary mics mentioned in this thread accomplish that by mounting the capsule flush
in the surface itself, pointing out. Both methods place the capsule fully in the boundary / pressure zone for the full audible range.
Boundary mounted directional mics may use different physical configurations (probably with the capsule perpendicular to the surface like you mention) which would produce response variations up top if their capsule diameter is large in relation to the wavelength of the highest frequency of interest. They are still using the same effect, just less effectively,
up to a lower effective frequency.
Still that's not the case with any of the boundary mics mentioned or pictured here so far.
[edit- oops, just went back to check the earlier posts in the thread. What I said above holds true for the Neumann and the DPAs, but it appears from the photo that the Schoeps BLC mount does position the capsule perpendicular to the surface instead of flush in it. Not sure about the dedicated Schoeps boundary mic mentioned but not pictured. The design tradeoff Schoeps made with that mount was the ability to use existing capsules. I trust their judgement.
In the case of the DPA mini mic mounts which also use existing capsules, the mics are tiny enough to mount inside the bulge in the rubber boundary surface so the opening on that surface blends seemlessly into the larger surface on which it is placed.]