If there are no inter-molecule collisions, there is effectively 0 pressure. (Ignoring the pressure that the walls of the container "feel", which is presumably not being modeled)
But that is exactly what pressure (of an ideal gas) is? No intermolecule collisions are required to model the behavior of a gas as we know it.
I'm not 100% sure if fluid dynamics uses a subtly different definition (though how hard the medium is to compress is a fairly universal definition).
Obviously thermodynamics does break down if you've only got a small number of particles, but then 'pressure' also kind of stops working as a concept when you've got mostly vacuum.
Roughly speaking if you put a bunch of bouncing particles in a box then even if they don't hit each other they're going to hit the walls of the box more often if you decrease the volume. This means pressure is proportional to density so inversely proportional to volume i.e. pV is constant.