You are correct. But here's the point that I think is being made. 'Back then' the teams in the metro areas were, in theory, constrained by district boundaries. A kid could only go to Central if they lived in the Central district (again, in theory). So, Central's talent pool was limited to the kids who lived in the Central district. Just like Pahokee's talent pool was limited to the kids who lived in Pahokee's district. Now, with school choice, and as a practical matter, Pahokee's talent pool (and the talent pool of all the other rural schools) is essentially still limited to the kids who live in that relatively-sparse geographic area. Contrast that with Central, who now has the ability to legally pull in any kid from Dade County. Yes, they are competing against other schools, like Northwestern, for that top talent, but recent history seems to suggest that only a small handful of teams are going to end up with the vast majority of the top talent.
To oversimplify, if one rural high school football team has a thousand kids to pick from (including gamers, trombone players, theater buffs, etc.) and a metro high school football team has a hundred thousand kids to pick from (because district lines really don't matter any more), which team is more likely to end up with a group of thirty talented football players? Now, it wouldn't surprise me at all if a rural team, like Pahokee, could beat the bottom-feeder teams in the metro area, because all the talented kids from those metro teams ended up at the super-power de jure. But when it comes time for playoffs, the rural team likely won't stand a chance against the metro power.
And, yes, there will be exceptions along the way, but for the most part, the numbers won't lie.