It’s essential, though the Bullet Journal spells out nice, thoughtful, and adaptable techniques. https://bulletjournal.com/pages/book
The reviews have caught hundreds of important ideas and tasks that had slipped my mind.
It actually looks like a typo as the sentences reads much more logically as "It's not essential, but ... "
FWIW I keep checking in on the Bullet Journal blog once a year or so to get some ideas, but I started keeping a variant of Bullet Journal about 5 years ago.
It's definitely been useful for not forgetting upcoming events, reflecting on the last month and for tracking trends on events in my life (e.g. sporadic food logging when making more of an effort to lose weight). One of the most positive outcomes was from recording key details about events - e.g. if I've met someone new at a language exchange, jot down their name and a detail or two about them. I found just re-reading these notes a day before I'm likely to encounter someone again has massively helped the next interaction because I'm not just going "Oh, what's your name again?"
Because there is a whole industry around hyping PKM systems and note taking systems with pretty weak evidence to support their actual utility. Calling them essential creates the impression that those not using them are deficient.
Yet, given that people are both massively productive with and without these systems, it really calls into question the effusive marketing and community promotion of these tools.
I use obsidian everyday, but I recognize that I could do just as well with some other tool. It's preferential, not essential.
Exaggeration is not necessary for communication of value.
For what? Many people do fine and much better than fine without.