The statement, “A round peg fits through a round hole” is partly subjective, so it’s not a counterexample to what I was saying about so-called natural harmonies. The peg physically and objectively passes all the way through the hole, but to say the peg “fits” through the hole is to presuppose that that endpoint (the object physically passing all the way through) is the desirable one.

The sense of fitness here is “to be of the right size or shape, as a garment for the wearer or any object or part for a thing to which it is applied.” It’s a question of suitability for a purpose. The purpose is the subjective part since that’s what makes certain features right or wrong for the job at end, while the suitability of those features may be largely objective.

You can tailor means to an end, and we can ask whether certain means can objectively achieve the objective. But the having of a purpose in the first place depends on a subjective choice and of an application of an ideal. The ideal with respect to the round peg is for the object to pass all the way through the hole. It’s in that sense that a cube won’t fit because it’s unsuitable to achieving that desired end. The desirability of that end is subjective, and it’s the same for natural harmonies. Vervaeke is presupposing the desirability of certain relationships.