Switch was actually performing its worst exactly 12 months ago (comparison wise) which was month 20. Then it gained a lot of ground during months 21 and 22. Now go back 24 months to month 8. Switch lost ground 2 years ago going into the holidays (months 9 and 10).
This is because the 3DS was extremely front loaded. It got a huge price cut in less than a year's time. So while 3DS + Wii U seems like it's doing better against Switch, it's only because, combined, they shot ahead quickly. Switch gained ground 12 months ago and it's total should even shoot ahead of 3DS + Wii U after this holiday season.
I do agree with this. The Switch in November and December is going to take the lead and never look back. The Switch is already roughly at half lifetime sales of the 3DS + Wii U.