To summarise, it’s the team that has been working with Nintendo for quite a while, but now they have formally announced it as a separate studio. Some interesting tidbits and history from the article:

The team has already been active for a number of years, and has worked on such titles as Super Smash Bros for 3DS & Wii U, Super Smash Bros Ultimate, Mario Kart 8, Mario Kart Deluxe, Mario Kart Tour, Arms and Mario Sports Superstars.

According to stats on the site, the team consists of 152 employees, 32% of which are new gradates. Around 40% of the team are artists, with the rest made up of designers and programmers.

Some job listing on the studio site:

Some listings are for “a 3D action game project commissioned by Nintendo” while another is for a game design position that involves “developing consumer games that will be commissioned by Nintendo”.