Their rationale will be the declining EV market in China (35% or so drop during 2019) which in turn was due to declining government subsidies. Nio had a pretty terrible Q3 and consumer confidence in China was falling as of Nov last year.
But, much of this doesn't impact Tesla given their cars will be cheaper now than they were at the start of 2019, and Tesla wasn't benefiting from Chinese government subsidies (but now will). 21k is an extraordinarily low and naive estimate - it will be at least 5x that.
According to the article, their theory is production problems, not demand problem.