Because it is relevent
And its not s a simple case of "your duties as a domestic client are automatically transferred to the contractor or principal contractor" and ths clients responsibility ending. Nor is it just a simple case of the contractor accepting liability. The client still has responsibilities under CDM.
You've obviously just read a few lines on the HSE website, so your ignorance is understandable.
The only good thing in the OP's favour is that the HSE most likely don't have the time or resources to do anything about it, and even more probablly can't be bothered to interpret their own badly written guidance.
But the most significant thing, is the risk not from the HSE, but from one of those workers going to a no-win no-fee solicitor, claiming exposure to ACM and then winning a tidy sum all because the OP client failed to ensure the necessary safety procedures were in place, making the site de facto unsafe and he liable as the client.