Super Mario Party, Pokemon Let's Go, Smash Bros, and NSMB U all released within a 4 month window during late 2018-early 2019. All of those games are in the Switch's top 10. Yeah, it actually was games causing Switch sales to grow.
As you said, except for NSMBU those were 2018 releases. And except for NSMBU and Mario Maker 2, all the high-profile Switch releases for 2019 were after the Lite was released in September.
Nobody has yet to satisfactorily prove that any one game (or four closely-released ones) can improve their system's baseline for many months on end. We've never really seen it with other systems before, so why assume it's happening now? Also, having good lifetime sales doesn't make a game a system-seller. Sure, Smash was an undeniable blockbuster and Pokemon Let's Go did very well, and there's solid evidence that both moved some surplus hardware. But neither NSMBU nor Mario Party appears to have moved the needle for software in the short term if what they did in Japan is any indication. Their initial sales in Japan weren't all that impressive either despite their solid lifetime sales; according to Media Create, first week sales for NSMBU and Mario Party were 166k and 142k respectively, relatively mediocre starts compared to a lot other popular games. In the U.S., NSMBU had already dropped out of the Monthly Top 10 in February and the YTD Top 10 by April, and Mario Party dropped out of the monthly Top 10 after the 2018 holiday season (Pokemon Let's Go's legs are hard to determine as NPD logged the Pikachu and Eevee versions separately).
You could make a case for January 2019 feeling some residual effect from Smash as there have been a few system-sellers to still have a small effect in the month after their release, but that's about it. Except for maybe Splatoon (and only in Japan), there's never been a single recorded case of any obvious increases in hardware sales two months after a notable system-seller's release. The effects of individual games on hardware sales are often grossly overestimated. Their effects are very short-term. And again, we don't see continued strong lineups over time result in growth in baseline sales for other systems, so why assume the Switch is somehow unique in that regard?
And FWIW, we didn't see a consistent increase in the Switch's baseline in Japan like we did in the U.S. While sales were up in January & February 2019, by mid-March it was down to being about flat from 2018 and it stayed like that until the Lite was released.