Was that how it went down? From what I remember Laura Kate Dale and Emily both were 99% sure that Splatoon would be a port, more specifically one named Refresh, and that it would come out way earlier in the Switch's life. I do remember that one of them created a Switch rumor chart on the day of the January reveal, and on that they just put "Splatoon", which seemed like obvious backtracking in case it ended up being a sequel (which people could tell it was as early as the October teaser where some of the models were different).
Maybe they predicted Splatoon before the October trailer, but even then that's one win and many losses on that particular game. Eh, it just seems like even the best leakers are pretty inconsistent. Nothing against them really, but I think people focus too much on the wins and not enough on the losses (or rather how general these wins can be).
It's possible that they assumed it was a port from what little they saw of it. Just like how Ultimate was originally rumored to be a Smash 4 port before eventually being shown as a new game. Like I said, Rogers can only go by her sources, and they may not always tell the whole story.