When anyone makes a claim for the lack of need of a Protectant on any carpet, there are certain assumptions implied.
Polyester (PET), PTT Polyester (Smartstrand), and Olefin marketing often purports rightly that they are largely immune form certain staining issues that would ordinarily be solved by the addition of a protectant on other fiber types. However, they ignore that soiling and staining of carpet is not all the same.
Polys and Olefin without a protectant are all subject to dry soiling, and oil based staining, as much, if not more than other fiber types. Polys and Olefin are particularly adept at absorbing non-polar oils and greases, for which a protectant could afford protection.
When these products are marketed as "indestructible", the conditions presented are usually well controlled and devoid of soiling circumstances for which unprotected Poly and Olefin are vulnerable.
The manufacturers lone argument is not to classify oil spots and dry soiling issues as a "stain", per se. So in this perspective, the product indeed "does not need a protectant". However, we know the devil is in the details. A protectant indeed can have a demonstrable value on these carpets.