Usually a pretty common mistake. Fluid is always compressible (else sound won't travel in STP water). It is whether one chooses to model the flow as compressible or not.
I think that is a simplification of the model for high school level physics. It's like when we use Hooke's law to describe a spring constant that only holds true for part of the range. Compared with a volume of gas, fluids are "incompressible."
