I am wondering if the information or guidelines in Advisory Circular are mandatory?
Certainly not mandatory.
However, the FAA will accept the use of information in an AC. In respect of certification matters, the applicant doesn't have to use the AC but, if not, then expect to have a somewhat harder time convincing the FAA that your stuff is good. In general, unless there is some very good reason to do so, an applicant would usually follow the AC guidance.
It is important to realise that what you or I might think the rule means over an ale or a coffee is totally irrelevant other than for interest. What counts is what either the Regulator or a relevant Court determines.