What Should Happen:

as long as one player has nintendo switch online, all other  players should be able to connect with them, either through other accounts, or by using the skip account selection feature to play as a guest of the primary account. When selecting Skip, the second player is still using the 1st players account, meaning it DOES have access to nintendo switch online in it's own right.
Overcooked works online with all players under the same account (there is no way to play with multiple accounts on that game anyways) and your own minecraft nintendo switch edition works online not only with second player using skip(guest of p1 account) but also with a second player using an account  without nintendo switch online so I know this is how that is supposed to function (tested today using online mini games).

What is happening:

When one player (1st player) starts up a world using the online mode (not just for multiplayer, required for achievements as well) a second player who is either using a nintendo account without nintendo online or connected using the guest feature of the 1st player (who does have nintendo switch online) cannot play splitscreen but erroneously recieves the message
"You are not connected to Nintendo Switch Online services. You will need a subscription to  the nintendo switch online service to play multiplayer. This can be purchased from the nintendo  eShop. Multiplayer can be disabled on this world in Game Settings."

This bug that stops players from playing online splitscreen when one has nintendo switch online is then accompanied by a few other bugs.

Pressing B while this message is present will get rid of the message but leaves that half of the screen black until B is pressed again.

The message indicates that multiplayer can be disabled on the world from game settings but the option to do so is greyed out and unavailable.

This is a major issue for nintendo switch. It is a system designed arround splitscreen multiplayer at it's heart, and not being able to play splitscreen with a guest in online mode or while playing online crossplay is deeply frustrating.