Sure they can. Even non-canon stories can have an internal consistency and continuity and be retroactively changed, scraped or rebooted.
But no one said fan ideas, reboots, retcons or reimaginations are "canon" or official continuity. No non-canon story, even one that is entirely consistent with the existing canon, is part of that canon unless approved. We all agree completely. It's not that fans cant retcon or reboot. They cant even create an official canon consistent story that doesn't reboot or retcon anything. Official being the key word.
But you can write a story cant you? And if it is written to be consistent with the Prime universe continuity you can say the word "continuity" in that sense. Why? Because the story is set there and is consistent with it. You can also write a story that fundamentally reimagines and reboots the entire Trek universe. Reboot is the correct term for that type of story. Or you can keep the basic continuity but make retroactive changes to it. You can retcon. If you want to withhold use of the word "retcon" for that type of story until it is greenlighted, flimed and aired and thus becomes "canon", that's up to you, but that isn't what a retcon is. Nothing to do with "official".
It was a retcon before it ever was approved. It differs from stories that do not retroactively change anything, nor is it a reboot. Stories that reboot are definitely reboots. A non-canon reboot is still a reboot. As I said earlier, call it proposed retcon, if for some unknown reason it makes you feel better. But retcon and reboot do not = "canon".