I believe they are trying to say that they made their movie off of what was only home movies.
Because: they had made- they made
from what was to have been- from what were
strictly home movies- only home movies
New sentence: They made from what were only home movies
**We changed the "was" to "were" because "was" had "to have been" in front of it so it didn't need to match "home movies" since "have been" matched it however, since I took out the "to have been" the verb became closer to "home movies" and we need to match "home movies" and "was"
***I tried to break it down, just remember that past/present/future perfect can, most of the time, be conjugated in simple past/present/future
Hope this helps^^ Have a nice day!