Again, Carlsen and Firouzja logged more than 700 hours last year playing blitz and bullet across all platforms. That's nearly two hours a day.
Anthony Saidy and many others have pointed out that experience in many different types of positions is essential for becoming a strong master.
I was brought up in the old school where we were all told not to play too much blitz or bullet. But there's more than one way to gain experience and become a world class player.
Carlsen has been training with Simen Agdestein and various other GM's since he was a kid - and back trhen he was certainly NOT playing bullet- he is born in 1990 while bullet appeared as a form of chess he was World's #1 rated player already.
And Firouzja ows a lot on his improvement to GM Ivan Sokolov who had trained himself and a few other Irianian telents (Maghsoodloo, Tabatabaei) since late 2015.
Sure, there are several ways to become an elite player, and speed chess is NOT one of them.
Play Longer Time Controls...
Here's what IM Jeremy Silman, well-known chess book author, has to say on the topic...
https://www.chess.com/article/view/longer-time-controls-are-more-instructive
And Dan Heisman, well-known chess teacher and chess book author…
https://web.archive.org/web/20140627052239/http:/www.chesscafe.com/text/heisman16.pdf
and the experience of a FIDE Master...
https://www.chess.com/forum/view/general/how-blitz-and-bullet-rotted-my-brain-don-t-let-it-rot-yours