I always thought this is a weird ask. Like asking the current employer to write a reference letter to the new employer, especially before the new employer offers the job.
The more bizzare a thing is, the less mysterious it proves to be.
Suggest you to call ESF admission office to check. Last year, my friend did check with the admission manager and she replied that the reference letter could be submitted later until she got the offer. This was not a must during the application stage.
But to play safe, better to check directly with the admission office again this year.