I'm not so sure; if there's a completely fresh batch of U20s each year (or two) and the senior international side consists of players age 22-35, one or two from each age group side becoming seasoned internationals would be about right.
Plenty of other players on that list have gone on to play Premiership standard rugby..
Selection for U20 squads must involve tossing a coin sometimes, meaning that some players who develop into top performers may miss out. We see it with the senior side, where some people favour a player who doesn't make the squad.
Some players develop late, but become world-beaters; others develop early but never kick on.
I'm sure there have been some whose career (leastways at top domestic level or above) has been curtailed by injury... or a career outside rugby opened up and they left the game.
All of the above also applies to the sides which the U20s play, so both the personnel and relative quality of teams can change dramatically. England suddenly picking exciting wingers and giving them the ball is a great example of this.
Then there's 'going on to play for another country'... especially where U20 seems to capture precisely no-one.