Acrylic Tub on wood subfloor and Mortar bed

I have a 50 pound bag of structolite,( a gypsum and perlite mix) as recommended by my supply house. I intend to lay a mortar bed under my new Acrylic Whirlpool tub. It is a Kohler tub, the installation directions for the tub specifically say

.."Do not use gypsum cement or drywall compound for this application, as they will not provide an acceptable durable bond"

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. I could venture to say Kohler is referring to wall spackling/patching compound, and are not ruling out all gypsum based products.

After researching structolite, it seems that it is a common practice to apply this stuff as a tub bed. My supplier says that they even get it under the feet of the tub, and rest the pump motor in the stuff. Of course, the dummies at Kohler do not advise on what to use.
Is structolite the best option??? Or is there a better material???

its what i use. easy to work. stays stiff. doesnt sag when mixed thick. i believe the bag says its a setting compound.