I think that British parents always want to move their children onwards and upwards at a young age. It's like collecting stickers.
In the USA, it is very normal for children to be 'held back' and spend an extra year in preschool, particularly late born boys. In some school districts it would be very unusual to see summer born boys in their chronological grade.
I think the preschool teacher is either being very astute, brave or political.
If she truly thinks your DS is too young for reception, she is really sticking her neck out, but presumably because she really cares about your DS. On the other hand, she may be filling you with gloom now, only for the school to add tremendous value over the next two terms and get your DS up to speed.
If you want your DS to move up, then that is final, but the school is showing you support in holding him back if that is what you want.
Any decision on this should take place at a formal meeting, with teacher, head of school, SENCO, both parents, and any advocate that you would like to have.