You are correct that there are some reasons to suspect that more buyers would have left reviews of his book than for other books. Even taking that into account 600 reviews is still a very high number of reviews and indicates a high number of sales.
Crisis of Conscience is the most high profile ex-JW book ever, people have also been highly motivated to praise and promote that book, it has been around much longer than LE’s book, and yet it only has 35% or so more reviews than LE’s book on Amazon. (Plus another factor, for people who know Amazon well, is that CofC has been around since the beginning of Amazon, when writing reviews was much more common than it is now. By 2017 purchasers were less inclined to leave book reviews than they were in the early days.)
Another factor is that LE’s reviews count has been artificially suppressed because he, or someone acting on his behalf, has succeeded in getting many negative reviews of his book removed from the site over the years. So the actual number of reviews left for his book is higher than the nearly 600 reviews that remain on the site.
He has many more reviews of his book than other near competitors - Eric Wilson has 65 reviews for example. Don Cameron’s book, which was very popular at one time, has 100 reviews. Neil Gardner, a more interesting YouTuber, and probably much better writer than LE has 84 reviews.
I doubt his sales are as low as 10,000 but I can’t prove you wrong. Even if that is the ballpark figure it’s still a phenomenally successful self-published book by any standard.
Again, I’m not arguing it’s a good book. The parts I’ve seen are crap and people who think it’s any good are dumb in my opinion. But the idea LE needs to take lessons on writing a book that sells is for the birds. If the argument is that it’s a crap book despite its success then I’m on board with that. But I don’t think he cares about that.