Odds are the entire event would be under NDA if that were the case. Nintendo almost never publicly tweets about NDA-related events either, which further suggests there's nothing new there, besides delicious looking cupcakes
Considering the tweets are generally drumming up the hype for the info dump tomorrow I really don't think that necessarily matters. We got several tweets from sources about the last Splatoon event but it wasn't publicized mostly because the info from that was tying into the general Nintendo Direct that we got a week later.
Either way, the screenshot doesn't tell us anything about what modes are available to try, the maps they're showing or whether there's new single player levels. Not that it necessarily matters.