Forget bullet, it's a joke.
Whenever someone makes a disproportionally bold and dismissive statement such as this one, you can almost always be certain that they themselves are equally unskilled in the area they are disparaging.🙄
You can learn from all aspects of chess which include all time controls.
In my experience, as you get stronger and more comfortable playing, you'll naturally gravitate towards Blitz time controls, as can be observed with the majority of advanced players on this platform.
Sure you can very slowly get better playing very fast chess but you can quickly get better playing slow chess!
Of course it's always easier if we ignore all those people who start out playing a slow game then, after spending an hour of their life losing just lose interest in chess completely!