Probably the most influential chess coach and author, Mark Dvoretski, was not a GM, and several good authors were not titled players, but had a reputation as strong analysts (often players without time or money or the health to play tournaments and often using their skills to help IMs and GMs in their preparation, or strong correspondence players).
The idea that only a super GM is able to write an useful chess book is similar to the idea that only former tennis pros will be good tennis coaches - but some famous tennis coaches never played a single pro match.
It could be argued that great chess players don't necessarily make the best teachers or authors. There are a list of reasons for that. Great learning ability and understanding, doesn't necessarily translate into great teaching ability. There are as many bad teachers as their students by percentage. They may be a bit stuck on themselves and put their own ideas ahead of a more sound approach. That could be debated and I am sure it has regarding other chess books. Also, a top GM would perhaps feel a bit in need to be secretive. They wouldn't want to publish their MO. Also there are linguistic considerations. A great chess author might only be only be a CM or FM, but can speak French, English and Russian, so they have a way of articulating things in a more concise manner. Even Lyudmil himself struggles a bit in this area. Perhaps some of these reason working together is why Magnus Carlsen seemed so elusive or poorly spoken in post game interviews about his games of positions from those games. A great talent like Carlsen might even play on instinct sometimes and not even know best how to explain his rationale for his choices. Many humans don't understand the best engine moves. If that is what it takes to be really good at teaching, perhaps this is an advantage Lyudmil has ?
Probably the most influential chess coach and author, Mark Dvoretski, was not a GM, and several good authors were not titled players, but had a reputation as strong analysts (often players without time or money or the health to play tournaments and often using their skills to help IMs and GMs in their preparation, or strong correspondence players).
The idea that only a super GM is able to write an useful chess book is similar to the idea that only former tennis pros will be good tennis coaches - but some famous tennis coaches never played a single pro match.