When it's not being used as a proper name, the word "harmony" is usually used in reference to music. Therefore, when you used the word, you connoted the musical context.

There's a difference between not intending the musical context and not admitting that your meaning depends on that context. The latter tapdancing would require equivocation, a covert reliance on two different meanings.

Not all metaphors have to be objectively grounded. On the contrary, poems are full of subjective metaphors.

I notice that you still haven't defined the objective, realistic sense in which you mean to use "harmony." Suppose you meant to say there's a nonmusical agreement that can be established between people and nature. (The first meaning of "harmony" on dictionary.com is "agreement; accord; harmonious relations.") Yet the top meaning of "agree" is "to have the same views, emotions, etc." So it's not at all clear how there could be any agreement between people and something which has no views or emotions.

Once again, this would be a metaphor that depends on the social context. People can literally agree with each other, and to say that people can agree with nature is to personify nature (as in the theistic or animistic sense) or it's to compare nature to a person, to speak poetically. The poetic discourse wouldn't be objective or realistic, but an expression of the speaker's feelings.

When Vervaeke speaks of cosmic harmony, he's assuming Neoplatonism, which takes mentality or some normative properties (beauty, godness) to be metaphysically fundamental.