1. Specifically the guidance "allows" but does not demand that employers can do this
2. Specifically says "may apply" , not does apply
In the actual pdf https://www.irs.gov/pub/irs-drop/n-20-65.pdf it is quite apparent to myself, as well as respected news sources, that the employer is still and ultimately responsible for the payment of the deferred taxes
So by "refuse to participate" do you mean that you will continue with the regular withholding? If so, what will you say when employees complain about not getting the "vacation" from social security tax withholding? I could see explaining that they would have to pay double tax (essentially) for the period after the first of the year, but some employees might rather have that?