Take your own advice, that article concludes with "Patent and copyright attorney John Arsenault told Fstoppers that New Portraits, although it looks like a case of outright plagiarism, might be a little more complex if it were argued in a legal context."

Meaning it is (or was) not, at the time the issue arose.

Additionally, in a legal context it is "more complex" because the artist marketed these as vignettes of his Instagram feed. To prove copyright infringement, one would establish copyright ownership of the artist's feed. Who own's a particular user's Instagram feed -- the user, Instagram, or is it shared copyright of all entities listed within the feed? To enforce copyright the latter would need to be true since the artwork is marketed as part of a social media feed.