[CHz]

So SuperSweep were in fact the ones responsible for Tetris 99: http://sweep.co.jp/index.php/20190603/

They haven't revealed the full staff yet, but so far these folks have claimed responsibility:

• Takahiro Eguchi: BGM
• Teruo Taniguchi: BGM, sound effects, voices, possibly the "Flight of the Bumblebee" remix based on him linking a cover of it
• Fumihisa Tanaka: unknown (it's just listed in his bio)