The entire movie seemed to be something that Shizuku from the Whisper of the Heart would write. I personally think it would be really cool if it was that way, a nice spinoff.
I don't believe that this is the case, at least it's not the story she wrote in the Whisper of the Heart.
As you remember in the WoH when old guy tells Shizuku the story of Baron and the his Mistress in Germany, he says that the they would meet again if humans met againg - and Shizuku said something like - "just like in my story!".