You are incorrect. The Premier League's exact criteria is as follows:
"Players listed on the club’s squad list who are still available to play in the match, including appropriately experienced Under-21 players (which includes Under-21 players who have played for the club, another Premier League or EFL club, or an overseas club in the current season. However, any Under-21 player who participates in a club’s FA Cup Round Three match and has not played in any other first-team competitions for the club this season will not be considered by the Board as appropriately experienced)."
They don't consider regular academy/U21/reserve appearances as 'first-team competitions', therefore under-21 players with no first-team experience do not count towards the 14.