The example of a person blowing into a tube while swimming deep in water is a nice argument for your case because everyone can relate to the fact that, as one goes deeper, one has to blow harder to get any bubbles to emerge.
However, bubbles or not, the over-pressure (that above atmospheric) is still transmitted to the fluid according to Pascal‘s law.
In the case of a model engine, running under prescribed conditions, rpm, etc. the pressure tap in the exhaust will have a given amount of over-pressure, no matter where the other end of the tube ends up. If this is, say 2 pounds at the muffler, it will be essentially 2 pounds on the other end of the tube. If 2 pounds is not enough to make bubbles in a deep tank of fuel, this doesn’t mean that the 2 pounds of pressure just disappears or goes somewhere else. It is still transmitted into the system. The air in the tube, and the fluid in the tank still transmit this pressure along. Of course it adds to the pressure in the bottom of the tank caused by the fluid “head” of pressure. This is a classic example of Pascal’s law which
I would suggest reviewing at the following link. http://www.engineersedge.com/fluid_flow/pascals_law.htm
The nice thing about this web page is it shows the pressure effect (equal on all surfaces) and shows how it combines with the pressure head effect.

To help visualize your example, here's my sketch of the situation you have described, where the over-pressure in the tube is fixed at a limited amount and insufficient to force a "bubble" into the fluid. Still it manages to displace some of the fluid in the tube, forcing the top surface of the fluid to rise, transferring the pressure to the air in the top of the sealed container. This pressure transfer is instant, complete and absolutely one-to-one. The pressure measured by the gauge at the top of the tank will MATCH the pressure at the input tube (at the RED arrow). The pressure measured by the gauge near the bottom of the tank will match the internal air pressure from above PLUS the fluid pressure caused by the "head" of fluid (it will be higher than the internal air pressure by the amount caused by the pressure head of fluid).

The reason that the companies I mention don’t suggest the uniflow design is not tradition but what they have found to be true and valid.

Regards,
Allan