I’m not familiar with the product nor the prohibition to use it. But I wonder if that explanation was a misunderstanding of the facts.
Gypsum is widely suggested to improve drainage in clay soils but the only condition under which gypsum is useful is when excess sodium is present in the clay. The good news for gardeners here in the northwest is that the clay soils so common here are not affected by sodium.
Clay soils become very hard when dry. So the key to successfully creating a garden bed for flowers or vegetables in clay soil is to work the ground when it is moist. Under natural conditions that typically occurs during May or early June. (At other times of the year, you may need to irrigate the soil before you dig; with clay, several on-of cycles works well to help the water soak into the ground,) Then, amend the clay by mixing in a least two inches of compost, level the bed, and seed or transplant. Repeat the amendments every time you put in your seasonal plants.
Please read “The Myth of Gypsum Magic” https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/gypsum.pdf
Also see “Improving Garden Soils with Organic Matter” http://extension.oregonstate.edu/polk/sites/default/files/MG_Handouts/ec_1561_improving_garden_soil_with_organic_matter.pdf
If you have further questions, please ask.