These words indicate that what the Apostle set forth above is a matter of custom. If anyone wished to argue with this, he would be reproved not by the Lord's commandment, compare 7, 10, but only by existing custom. Customs change.
In particular, concerning men growing their hair, the Apostle finds that nature itself teaches that this is a dishonor for a man (v. 14). Here too he refers not to the Lord's commandment, but only to "nature," considering long hair natural not for a man, but only for a woman (v. 15).
Yet naturalness is not always an obligatory property of human actions. In other words, not everything that is natural is obligatory. For example, it is natural to go barefoot in hot weather, but it is not obligatory.
As for covering the head, there is a distinction between simple covering and crowning the head. The miters of Orthodox bishops, and the kamilavkas and skufias of priests, are head ornaments, like diadems or crowns, that is, bands or wreaths.
The Apostle could not have condemned such ornaments. As a zealous visitor to the temple in Jerusalem, he of course saw the headpiece, in Greek the mitra, on the head of the Jewish high priest (Ex 29:6; Lev 8:9). He also saw head coverings on the priests (Ex 29:9; Lev 8:13).
Therefore he could not have thought that every ornament or crowning of the head was contrary to the commandments of God.