It might sound dumb, but I've heard this is true.
You don't need a Nintendo Switch Online account to play local wireless.
You can see in Nintendo Support with two examples:
For Pokemon Let's Go:
Select “Nearby player” to connect via local wireless or “Faraway player” to connect with another player online.
A Nintendo Switch Online membership is required to connect online.
It specifies that you need an account only for online, it never says anything about local, if it was needed it would say something about it.
Another example could be with Super Smash Bros. Ultimate
If you check the Online Play section it says:
From the main menu, select “Online.”
Note: A Nintendo Account with an active NSO membership is required to use this feature.
*NSO: Nintendo Switch Online
Meanwhile in the Local Wireless Play section states as following
- From the main menu, select “Local Wireless” from the list on the right side of the screen. (The icon looks like a Nintendo Switch console.)
- One Player (Player 1) needs to create a Room.
It doesn't have a note telling you that you need a NSO account, if you needed it would be stated as in Online Play.