That is not true in Spain where you need a Spanish licence to skipper a Spanish flagged boat. That is a big barrier to a non Spaniard who wishes to buy a boat in Spain, hence the popularity of registering under a different flag.
My Spanish friends who lived in the UK moved back to Spainand have their boat under British Flag and haveUK RYA qualifications.Latest news is they are being find for not registering their boat on arrival.Under the law they can use the boat as is.