See this is where the "structural advantages" of existing racism play out.
Jerry Rice was born to a brick mason in the early 1960s - when not only states like Georgia eliminated their public school systems to avoid integrating them, but when a black man had NO CHANCE at being a HS Football coach, much less a collegiate coach
John Elway - born roughly at the same time, was born to a football coach
Elway talks of how his dad prepped him to play football. Something that Rice's dad could not because of racism.
Rice developed his natural athletic talents as a runner, but only was drafted into playing football in HS, where he was slotted into a role that his talent made him easiest to train for - running back.
And the rest is just the momentum of being slotted and rolling forwards.
This is a quintessential example of how structural racism of 40 years ago manifested itself into differentiated outcomes some 20-30 years later. And that in turn sets up the "preconceived notions" of who is good at what.
Note that "preconceived ideas" are really just another way of describing "prejudgements" - which is a more polite way of saying prejudice.
Now remember, the guys who are coaches today, are often the players who came up in the Rice/Elway generation. So their "preconceived ideas" are going to color who they recommend to try out for what positions. And sure there will be exceptions who fight that and who try out for other positions and shine - but the Warren Moon's prove by the extra work they have to do that there IS a racial component to those "preconcieved ideas".
Now you may try to argue that over time, the HS coaches are going to let go of their pre-conceived ideas in the quest to win - but remember, most HS coaches see their roles more as educators than has having a mandate to win. And they see new kids every year, most do NOT have "programs". Thus their "preconceived ideas" which are part and parcel of the "values" they teach, get perpetuated forwards for a long time.
That means that while the racism isn't as strong as that which precluded Rice's dad from being a coach, it still exists. And just as that overt legal racism took 20-30 years to manifest its outcome, so too the "preconceived ideas" of today's HS coaches will perpetuate for 20-30 years as well.
BTW, the reason NFL roles tend not to be studies is because they are so specialized you really need to go back and run a prospective study starting at the youungest levels and looking at a cross section of the whole population. That sort of study is extremely complex and expensive - not one likely to get funded.