💡 Detailed Explanation
「～にほかならない」is used to strongly assert that something is exactly that and nothing else. Here, it emphasizes that his remarks are definitely a sign of dissatisfaction. 「～に相違ない」 expresses strong certainty or deduction ('must be'), while 「～に尽きる」 emphasizes that something boils down to or is nothing more than its essence. 「～にすぎない」 is used to dismiss something as merely that.