Can anyone really know how an ATS ranks candidates?

Not in detail, unless they work for the vendor or have access to the employer's settings. Vendors generally do not publish their matching methods, and employers choose which features, filters and questions to use. Anyone outside that process is making an educated guess.

What can be said with reasonable confidence:

• Systems need to extract text from your file, so text-based files and simple layouts are safer.
• Recruiters commonly search by job titles, skills and qualifications, so clear wording helps you be found.
• Screening questions can filter applications, so accurate answers matter.
• A person usually makes the final decision, so the resume must persuade a reader too.

Be cautious of anyone who says you will definitely pass an ATS; no one can honestly promise that.