Seems plausible to me...
Server sees player A wants to get in and checks the number of people in the intance, it is <25 so the player is allowed in.
Server sees player B wants to get in and checks the number of people in the instance, server lags enough that player A is not in the list of players in the instance, it is <25 so the player is allowed in.
It will probably work better on higher pop and more laggy servers.
An idea that might make this easier:
Make a raid of over 30 people.
Have them all /follow a person not in the raid.
The person not in the raid runs through the portal but of course doesn't get let in.