I agree with the agents or editors who wrote that.
For my books, I’ve done tons of research, and now know a lot about art forgery, TB asylums, murder methods, ELF, and blindness. The research makes the book richer and it also gives readers a two-fer: an engaging story line coupled with some new facts that they can file away about how things work in the world.
Even if we write fiction, I think we have a duty to get it right, at least as right as we can. Sure you can get bogged down in details, and do so much research it overwhelms the book. But I think fiction should not give readers an erroneous impression that they are learning about something, when really the author made it up out of whole cloth.