Not sure of the exact interpretation, but from the Moscow 2013 entry standards...
Performances achieved in mixed events between male and female participants (see Rule 147) will only be accepted under the following circumstances:
o For all field events and races of 5000m and over held completely in the stadium the results will be:
1. Automatically accepted if achieved at National Permit competitions
2. Accepted only with the Area Association's approval if achieved at competitions under IAAF Rule 1.1 (i) and (j)
3. Never accepted if achieved at competitions conducted under IAAF Rule1.1 (a) to (h)
o For road races (marathons and race walks) the results will always be accepted
There is a difference between records, where mixed races are not allowed, and qualifying times, where at least in some cases they are.
To run a record in an all women race, the record breaker must take the lead at some point and win the race. In a mixed race, if it were allowed, the leading women could set a record without ever leading the race.
However, to run a qualifying time in an all women race, the runner can come first, second or last and it does not matter as long as the time is fast enough.
I'm not sure how much difference it makes if those other 15 minute 5k runners, are a bunch of Kenyan women in a diamond league race, or a bunch of guys running in Boston.