They knew it was going to be a success. Switch Lite was likely the original hardware, they must have started planning and designing it as soon as the 3DS was released but their plans changed drastically after Wii U bombed
Lite being released as early as 2019 severely indicate it was planned to be released just after the launch. If hybrid wasn't a selling point they would rush Lite release for 2018 and selling at 200 USD any problem with price would be solved because this was a proper handheld and Nintendo handhelds are successful, period
At worst Switch hybrid was destined to sell some times better than Wii U as Switch wasn't going to experience a short life span and the userbase, even if buying mostly for playing handhelds, would still buying the hybrid version. Is the worst case scenario Lite would outsell the hybrid eventually, but Switch family would hardly sell much worse than the 3DS
Of course what happened was the exact opposite, the hybrid nature was the main selling point fairly outplaying their brand and market domination with portable devices. I can't say this surprised Nintendo, at best we can say they were not sure about how the market would receive an hybrid as it was a risk with no precedence, but I guess after some months of 2017 adopters data they already knew Switch was going to be a massive success
