Smile when you say that, stranger...

You're not required to like 1632.
(Though many of us do.)
But the series exists because it is outright fun and because he picked a truly pivotal point in history to perturb, full of interesting historical figures and the time of the birth of the modern state. A fair amount of the "derivative efforts" in the GRANTVILLE CHRONICLES aren't even fiction, but rather historical analysis and discussion.
But of the fiction, given the official invitation and veting, I don't think fan fiction is a term that fits the 163x Project: it is closer in tone to the THIEVES' WORLD or MEDEA shared universe project in that a significant portion of the contributions are accepted as canon and incorporated into traditionally-published volumes of the series.
So, taste issues aside, I would suggest it does not fit into a discussion of fan fiction.