The way I interpret it is, memories and film shots intertwined. We also get a glimpse of Calum's struggle as well. You have to watch between the lines/clips to understand what's going on. It makes you oblivious same way Sophie remembers it. How well do you remember things when you were young? which core memory remains? Do you remember the struggles people around you?
Its refreshing to see this film way, but it took an afterthought to process it, I think you have to be a little artsy and also have a certain familiarity with the struggles to understand it. Its like user experience, how many users understand what is being expressed? so its not everyone's cup of tea. Does this type of movie affect most of the viewers? certainly not. But I like it the way it is.