Grimoires as a whole, overall, are not credible; point is that magickal workbooks are fraught with incorrect & misleading information.
From spirits and their attributes to how to summon them and general guidelines, at the very least part of this information is incorrect. This pertains especially to grimoires, since a lot of these are unverified and sources are difficult to get a hold of (it’s not like you can talk to King Solomon about the books attributed to His name).
Even worse, the average occultist or magician generally speaking blindly accepts this and rarely questions what he or she is reading.

Based on my knowledge & experience, most authors of these workbooks that date back centuries or longer even did so deliberately; general reason is that they knew that those that are worthy of this information and power would, due to their own research and perseverance, come to this realization. So a test to filter or weed out superficial interests from those that don’t have what it takes to inherit said power. See below video for more.