By saying that two variables are directly proportional, it is saying that the two variables have a positive, linear relationship. Just as Hooke's Law approximates that the extension of a material is directly proportional to the force applied to it, which is not true for non-hookean materials, Ohm's law approximates that voltage is directly proportional to resistance, which is not true for non-ohmic components.

V = IR and variants are used to calculate V, I, or R from a single set of data; proportionality refers to a continuous function.

What is really being stated is that V(x) = IR(x) for a point x. This implies that current, I, is a constant, which we know is not necessarily the case.