quoting: "A few examples of protected concerted activities are:

Two or more employees addressing their employer about improving their pay. Two or more employees discussing work-related issues beyond pay, such as safety concerns, with each other. An employee speaking to an employer on behalf of one or more co-workers about improving workplace conditions."

Would that letter fall under that? I think there is at least a somewhat credible claim it could (and also a credible opposing counterclaim that the form of speech was meant to be defamatory/disparaging, and not a protected activity), but I am not a lawyer.