1. There are "fitness influencers" on instagram who are trying to build a career out of this
Ok. There are also software bloggers who are using their blogs to build up their tech careers. And startups who are using their blogs for marketing/PR purposes. I wouldn't go around accusing Robert Martin of being "full of shit" just because he wants to build a career out of the things he writes about.
2. There are some "fitness influencers" who haven't earned the credentials needed to be taken seriously
Sure, but that's the whole premise of social-networking platforms as opposed to traditional media. It seems a stretch to accuse everyone of being full of shit just because they chose to build their brand using social-media. What's next: "Everyone on the internet is full of shit"? "Everyone on Hacker News is full of shit"? "Everyone writing articles on random sites like MelMagazine is full of shit"?
Personally, this whole article reeks of clickbait and faux-outrage.