I had to look it up myself a while back.
A homegrown player is defined as one who has been registered with an English or Welsh club for a continuous period of three years before their 21st birthday. Additionally, each club must name at least one club-developed player in their matchday squad, or else they must name one fewer player on the bench. Players under 21 are exempt from these registration rules. So I'm guessing this is why Sutton will be here for a while.