I suspect this simply means building Y at Fremont has been decided. If so, any near term big changes there are probably simply adjustments to S/X production in order to open up some of the space needed for Y production.
Doesn’t necessarily mean timeline for Model Y has been pushed up.
For whatever reason, Bloomberg conflated this new email from Jerome with Tesla’s Q1 shareholder letter where they wrote that 2019 production might be as much as 500k.
That line re 500k is old news, put out while Model Y production guidance was for late 2020. I wouldn’t expect any Model Ys in 2019.
fwiw, I suspect the 500k mention in the newsletter was just to give Elon some cover in the midst of the SEC requesting that he be held in contempt for his earlier 500k tweet.