In the study of energy-preserving methods for Hamiltonian systems, polynomial continuous-stage Runge--Kutta methods play an important role. Necessary and sufficient conditions for such methods to be energy-preserving have already been established. They are energy-preserving if the matrix $M\in \mathbb{R}^{s\times s}$ defining the method is symmetric, and the converse holds under the assumption that a certain $s\times \infty$ matrix $Φ^\mathrm{CSRK}$ has full row rank. It was conjectured in Remark 3 in Miyatake and Butcher (SIAM J. Numer. Anal., 2016) that the full-rank assumption should always hold for every consistent polynomial continuous-stage Runge--Kutta method. In this paper, we prove the conjecture by showing that the matrix $Φ^\mathrm{CSRK}$ has full row rank under the standard consistency condition. The proof is a direct application of the polynomial moment problem solved by Pakovich and Muzychuk (Proc. Lond. Math. Soc., 2009).