It turns out that the reason why this is the case is because the Switch’s Online voice chat service is being hosted on Google’s servers. Given that Google services are still banned in China, it means that users attempting to access the Switch Online service will not be able to do so. However note that this is only for the console’s online service, which means that using the console offline and for single-player or local multiplayer games should not be an issue.
That being said, it is unclear as to what Nintendo plans to do about it, but it has been pointed out that since the Switch has not been officially launched in China, it seems unlikely that the company will be addressing it. There have been some temporary workarounds offered, so you can go ahead and try that if you absolutely need to connect to the service, which involves the changing of your public DNS.
Interestingly enough the Switch Online service isn’t the only gaming service to have been blocked. It was previously reported that Twitch has also been blocked in the country, despite it being available earlier.