Methanol in some substances does alter the chemical. Example-if you have sugar water and you add methanol and boil off the water you get anhydrous. First the sugar in water was safe, now its chemically altered and its now unsafe. This can be the same way with using methanol on certain substances, some are not affected and most are. Especially weak esters (the short ones) and most 17methyls.
The purification should be done at a lab, you can cut hormones but that's a different process and no its not adding bunk stuff to it (were not cutting coke here) Its the addition of HG purity with an assay test and cert to combine and elevate quality.