This could be still possible if he would document public API of his temporary closed source backend. Then anyone can extend portal but it is still copy-protected.
In such approach you have two instances of running api V1 - known publicly & frozen, second V2 without doc but used for dev and changes for the time when it becomes mature to document.
Yes,it may be an unnecessary overhead eating U time when things are changing rapidly.Best is to make snapshot & document it as v1 publicly, then do changes in v2 & documented it later.