It’s a theory. Most people don’t do a double 800-400 and even less do 800-200 because they are usually a member on their 4×4 team. There is a kid who recently went 24.2 in open 200 after doing 1:59.9 in 800 at the Urbana, IL High school invite. He’ll make State Finals, but he won’t win, because he lacks the overall speed required.
A girl recently did an 400-800 double of 58.5 and 2:20 for Bloomington, HS in Illinois.
What TP means is if your 200 is only so fast, then your 400 can only be so fast, if your 400 is only so fast then your 800 can only so fast.
You cannot expect a 25sec 200 runner to be better than a 54sec 400 runner.
What level? State Meet Finals level.