This is an oversimplification, in a way that is likely not obvious to a lot of people on this (software-focused) forum. An SDS does not have to list exact amounts, does not have to disclose some details of how an ingredient or mix of ingredients was processed, and (depending on jurisdiction) may not have to identify some "safe" ingredients at all. Some ingredients may be identified in relatively vague ways, that are sufficient for safety purposes but do not reveal the exact product. As the SDS you linked to says "The specific chemical identity and exact percentages are a trade secret". An SDS is certainly very helpful to reverse-engineering a product, but it doesn't tell you everything.
All that said, yes, the main strength of WD-40 is its marketing and ubiquity, and claims about its secrecy have more to do with marketing than anything practical.