Is that right? A 70% MoM drop for NS?
I get that March was a launch month, but it's STILL sold out here.
How sure can we be that NS isn't under-reported in April?
99% sure. NPD's retailer coverage is above 90%, so there's only a very small percentage that is estimated. In case Switch is undertracked by NPD, it wouldn't be by a significant amount, so such speculation is moot.
The reason why launch month was so much higher is that Switch production started in fall 2016, so the number of units that was made available in March far exceeded the amount of of one month's production.
A comparison with the PS4, another system that was supply constrained in the months following its launch, shows something similar. In the 1.5 months in 2013, a little over 4m units were sold. The following three months combined were ~3m.
Nintendo reported that 2.74m Switch units were shipped in March. Expect a similar amount for the three months of April to June, so the total should fall between 5-5.5m when Nintendo publishes their next financial results.