Can be, or will be? This sounds exactly like what was said about Amed. Potential is just that, potential...
legacy.baseballprospectus.com
Mets shortstop Amed Rosario was a plus defender on the rise when he sat down to talk at the 2016 Baseball America Prospect Pad.
www.baseballamerica.com
Alright, one last since you replied with what I expected you to reply with.
Amed has always been a plus vertical defender at SS. What made him scouted as a plus defender as a minor leaguer.
I myself scouted him as a plus defender, I bet I can go find my old scouting report card somewhere on my home computer. But that was because he was incredibly gifted going in and going back on balls, which most people agree is the harder skill to have than going left to right. He still is one of the best SS's in baseball vertically as well, especially on balls hit in front of him. But he was not a plus going side to side.
So much of your side-to-side range as a MIFer is footwork and a natural feel to read the ball off the bat. Amed never really took the steps there you would expect from someone with his speed. He is really good at reading balls hit right at him, but he struggles to read and react to the ball off the bat when it isn't hit right at him. Big reason why him in the OF failed as well.
But back to the source you are quoting. Assumption is a major part of scouting services. If someone has range coming in and back mixed with plus speed and a good arm, the assumption can be made that they are also going to be a plus defender going side to side. As a scouting service, like Baseball Prospectus, you are limited in the times you are able to see players. Which means if you don't get to see them make every play in game you have to assume using other data at your disposal to fill in the gaps on their missing traits you didn't see, or fill it in during warmups, which a lot of scouting services do. You are also extrapolating a handful of games you are seeing in person to come up with the scouting report for how that player is for the remaining 99% of their season.
And warmups are an awful thing to base scouting reports on, as are sim games when conditions are being controlled, but when you are limited in your access to players by not being associated with the team and needing to scout all 30 teams instead of focusing on one team, it helps to fill in the gaps. But it is assumption based.
It's why guys like Bieber slip through the cracks on these services. Or scouting reports are so outdated it doesn't even seem like the scouting service is describing the actual player you are wanting to read about. I have always said how flawed these services are, and I will continue to do so. Take all of them with a grain of salt. They don't have the manpower to effectively scout a player 100%, they just don't see them enough, and in turn have to rely on assumption or a controlled environment to fill in the gaps.
But Amed is and was a much better vertical defender than Arias, Rocchio is probably just a tick below Amed in that respect, but both are significantly better than Amed ever was at going to their left and to their right. So yes, all 3 are scouted as plus defenders. But the things they do as a plus are significantly different.