It was reported a few weeks ago, that Super Mario Maker 2 would not have have online multiplayer with friends. It turns out that the game will indeed have Online Multiplayer with friends but it will come in a future update. The Game’s director, Takashi Tezuka confirmed this statement during the Nintendo Treehouse Live | E3 2019. It was later tweeted on the @NintendoAmerica Twitter account.
Mr. Tezuka has confirmed that the ability to play online multiplayer with friends is coming to #SuperMarioMaker2 in a future update. Stay tuned for more information to come. https://t.co/oxGuEcJ8Om pic.twitter.com/1AfK1vEGIe
— Nintendo of America (@NintendoAmerica) June 11, 2019
Source: @NintendoAmerica