Any Christian church that is not Catholic is considered ‘Protestant’ as in Protesting against the Catholic version of Christian worship.
Most formally (versus functionally: lower-case “a”) Anabaptist denominations are not Protestant. They trace their history as always having been separate from the Catholic and Orthodox factions.
I am not Anabaptist, but I have been in ministry, and have studied Church History (at a private Christian school). Calling all non-Catholics Protestants is simply inaccurate. (Would you call Greek Orthodox Protestant?)
I have personally known knowledgeable Anabaptists who were incensed by that false characterization. They wanted no more association with Luther or Calvin than with Erasmus.