That was exactly the point. No data is sent to Google servers. A GET request is made to download a dictionary. As with all requests, certain information is most likely logged on the server (IP address, user agent string, URI).
In my tests (on Linux) I just turned off spellchecking and no requests were made.
I think that the default settings should be changed.
On the other hand I also get Laurent's statement.
I even removed the spellchecker icons from my UI on my macOS machines. But I am most likely not a good reference point anyway, because I use multiple security layers.