"Do you have a source for that?"

It needs to be stated the other way around. Since Life Costs are not typically tax deductible, the IRC provides for what is deductible, and doesn't include what is not. Knowing you cannot prove a negative, the real question would be:

Please cite your reference for: "add the non-deductible portion of Mort Int (Mortg in excess of $750K) & property taxes (SALT limitation) to basis when selling primary residences for gains outside of the home owner exclusion"

Because not finding this in the code isn't the same as finding this in the code.