It's completely wrong.
The 84% meme is confusion regarding the molecular weight of MDMA and HCl.
Pure MDMA (called the free base) is an oil. It is insoluble in water - making it hard to absorb through the GI tract. Like many medicines, it is formulated as the hydrochloride salt (HCl) which allows it to be soluble in water and absorbed easily by the stomach and intestines.
Pure MDMA-HCl can be found (really something like 95-99% MDMA-HCL 1-5% reaction byproducts that were left behind after filtering)
For every 100 mg of pure MDMA-HCl 84.7 mg is MDMA and 15.3 mg is HCl
All dosage recommendations are based on the MDMA-HCl weight.
What is important is that other salts can be used that have a much larger molecular mass than HCl -- and that is where you get effective differences -- but outside of a few places in Europe and Asia HCl is the salt used.
That said, if you look at pillreports, the mg dose is the amount of ACTUAL MDMA-HCl in the pill based on GCMS/HPLC (really precise testing of the component molecules)
as far as moon rocks -- most isn't MDMA -- but those that are can have a high degree of purity, very clear crystals/rocks have low impurities (else the crystal doesn't form well)