There's nothing concrete to say that "Nikon is here". They could be under the impression they are losing customer base to mirrorless, and by releasing a snippet, they are hoping it could stop the fence sitters from migrating away from them. It may be announced tomorrow, it may be announced in 3 months, it may be announced for release next year!
Nikon's teaser campaign is a reliable indicator that they are close to shipping — i.e. 3 months or less. Sometime in the next 4-6 weeks there will be an official announcement, then 4-6 weeks after that, the product will begin shipping. They just don't do teasers without a relatively short shipping window that they know they can meet.
So-called "development announcements" are different, and are what you are thinking of. Those are sometimes done to stop perceived market erosion, and they can sometimes happen many months before a product ships.
In this case, we have a teaser and a development announcement, but the teaser is dispositive. It means with 99% certainty that the product is relatively close to shipping. The production line is almost certainly already running.
Addendum: It's worth noting that Canon typically runs on a more compressed product rollout schedule than most other camera manufacturers, and they don't do a lot of teasers and development announcements. So, let's say both Nikon and Canon had a ship date of October 15. We might not hear anything from Canon until the official announcement 3 weeks before shipping, even as Nikon runs a teaser campaign for two months beforehand. I'm not saying one method is better than the other; just noting habits.