Okay, re-reading the article it is kind of confusingly written.
Has Pachter actually said Switch is headed for 13 million in 2019, or is that just the prediction of the article's author? And is the 13 million prediction for calendar 2019, FY ending March 2019, calendar 2018? It's all rather vague and poorly articulated.
Seems to be a yearly edition where a few analysts submit predictions for the upcoming year while reviewing their own predictions from the previous year. The author of the article doesn't have any input beyond the introduction for the article, so you are reading Pachter giving himself a pat on the back for no good reason.
Pachter says that his prediction that Switch won't sell 20m units in calendar year 2018 is worthy of an A+ grade because Switch is on pace for only 13m. Of course that 13m figure is nonsense because Switch will easily be above 16m, but that's Pachter for you.
The reason why Pachter chose 20m to begin with is that there were rumors in late 2017 that Nintendo plans to ship 20m+ units in the fiscal year ending March 2019 and an actual interview with Nintendo president Kawashima in December 2017 also had such a question; Kawashima answered something along the lines of it being possible that that will be Nintendo's actual forecast, but obviously did not confirm it at that point. While calendar years and fiscal years are different things, Pachter ran with the number because he loves to set up things in a way that Nintendo falls short. I guess at the start of 2018 he was pissed that Switch didn't sell at his predicted pace of 1m in the fiscal year ending March 2017 and 5m in the fiscal year ending March 2018.