I actually meant about how long we have known about it rather than being in development, if that matters any. Still I still think this is a lucky guess. If people did not know about it by January, at all, they probably would not say things like Trials of Mana getting in Smash Bros. Ultimate as some spirits, would they? Seriously, who could put into a leak a game that was not known about, anyway? And predicting Trials of Mana is anything but impossible. If it were, the game would not exist at all, or all of the data that even slightly hinted at its existence would never be found by anyone at all due to, well, impossible security, and it is impossible because if it were possible, nothing about the game would be leaked at all. That said, I think January is a good time to say something about it. The game was announced at E3 2019, after all, so there was plenty of time to think it wouldn't be a bad idea to include something about it and Smash in a leak one day.
But you see, calling things like this impossible is very much of a long stretch. I'd say it is very well possible.