I recently boosted my rogue who had zero shuffles played (or arenas) this expansion, and the system started me around 1850 mmr too.
That first lobby went 3-0 (then someone left), and then my next one was around 1980.
For those 2k+ starting mmrs I imagine that is mainly just carry over from season 1 mmr, but I could see people being 2k+ mmr after their first lobby if they do well.