As per my original comment, someone could agree that probabilities represent beliefs but also hold that the content of (“legitimate”) beliefs consists only of statements about frequencies. This would allow them to simultaneously hold a frequentist interpretation of probability and a Bayesian epistemology. (As you said, “There is no Frequentist epistemology, a frequentist could hold any epistemology they wanted more or less.”)
As per my original comment, someone could agree that probabilities represent beliefs but also hold that the content of (“legitimate”) beliefs consists only of statements about frequencies. This would allow them to simultaneously hold a frequentist interpretation of probability and a Bayesian epistemology. (As you said, “There is no Frequentist epistemology, a frequentist could hold any epistemology they wanted more or less.”)