But then there is so much wrong here it's hard to know where to begin. I'll go with the basics though: The article relies primarily on two doctors, one who is at odds with the consensus in the field and another based on an editorial, not a peer-reviewed piece of research.
In fact, peer reviewed research directly contradicts some of the opinions, such as stating it's unknown if SSRI medications raise or lower serotonin. It is known, they do raise it  (and many other pieces of research to support this)
Undoubtedly pharmaceutical companies have made quite a bit of serotonin and drugs that influence it, but calling serotonin a "myth" is, at best, overreach and at worst click-bait.
I'm sure it's comforting for the tweet author to pretend that harmful concepts come exclusively from outside his profession, but that's just not true.