The provided source doesn't support this claim. The only part which could even remotely be constructed as support for this claim would be 3 and the anecdata there include "He suspects" and "He thinks" - on the other, that you can end on such a list without knowing the reason (and often without recourse afair) is a real problem, but that's not what you claimed.
There are other abuses that we have concrete evidence of. E.g., refusing the cooperate with the FBI. (https://ccrjustice.org/home/press-center/press-releases/laws...).
Hell, up until 2005, the TSA used CREDIT SCORES as a component in deciding who should be on no-fly lists...
DHS does not use social media posts alone to place someone on watch lists, but they are incorporated into predictive assessments. Given two people who are otherwise equivalent, the one with public stated controvsial opinions IS more likely to end up on a no-fly list. By the govt's own admission.