Too much noise and nonsense around this. The policy allows athletes to qualify by time OR by rankings.
To fill the spots, the IAAF will need to read down the rankings lists to find *at least* the highest ranking that would allow them to fill all the spots, subject to national limits of 3 per nation.
This spreadsheet tries to help estimate this point.
So for 5000m, for example, USA should have a pool of at least 10 athletes to choose from (based on rankings today, which of course will change up to the final selection date cut off) who can then have a nice trial as you like to do.
Of course, then it is up to USATF and other NGBs as to how they implement this. But that's always been the case.