Update: Nintendo issues official statement to clear things up.
Update 2: Nintendo has issued the following statement to TechRadar regarding this situation:
As stated on the Game Store pages for the Pokémon Sword and Pokémon Shield games at www.nintendo.com, a Nintendo Switch Online membership is required to use the games’ online features. The games include an in-game option to purchase a Nintendo Switch Online membership for those who do not already have one.
Read the full article on nintendolife.com