On Switch, no game cards support writing of save data or anything else. It’s a departure from 3DS and all previous Nintendo cart formats for as long as games have supported saving at all.

That change is probably done to help tie saves to user accounts, enable cloud saves even when the card is not inserted, accommodate variable-size user data features like level creation, and mitigate the risk of game save based exploits like Twilight Hack spreading from user to user.

Unfortunately that (plus inability to put saves on SD card) means that backing up your own save data requires either being able to run homebrew on the system that has the save or having another Switch that can and relying on Nintendo servers to perform the transfer. Either way, having a Switch that runs homebrew means you don’t need this dumper.