Even if that is the argument being made, I don't think it's true. Rashford was the highest rated academy product in quite some time,
@roonster09 pointed some quotes out and there are others out there. I remember Ravel Morrison's agent saying Rashford was the best player in the United academy after the Morrison/Pogba generation.
You're never going to convince me of this because I have watched the u18s and u21-u23s. There is nothing in Rashford's performances that said he was a clear cert where as Adnan, Greenwood, Wilson, Ravel, etc... weren't. It's just making something up because hindsight shows us that clearly there must have been something that Rashford showed that others didn't that would lead to succeeding at the top level. There just isn't.
You're right that Morrison's agent said as much, but that was when Rashford was much younger. For example, what did Rashford show at that young age that Gomes didn't? Gomes was the more technical player who captained a really talented English generation.
Again, I'm not saying Rashford wasn't rated. He clearly was as was Gribbin. I actually thought Mitchell was doing really well for the u18s prior to his injury, though I don't think he was as rated as Rashford prior to the u18s. Not sure on this. The point is, that Rashford was a great player, but at the same time wasn't some standout where everybody thought he was the next big thing as if you couldn't make that case for Januzaj, Gribbin, Gomes, Greenwood, and Wilson. The latter part of that statement is hugely relevant and connected to the former.
Rashford excelled at the u18 level like all the other top talents did at the u18 level. You could definitively say or hope that United have a first team player within Rashford, but that would be the case as with all the other names I mentioned too.