Not the lawyer I talked to but:
https://myorlandoimmigrationlawyer.com/trump-administrations-new-90-day-marriage-rule-puts-green-card-applicants-at-risk
Under the 90-day marriage rule, if a foreign national enters the U.S. and marries a U.S. citizen or permanent resident within the first 90 days of being in the country, USCIS will presume the marriage to be a fraud and will deny a Green Card, claiming the applicant misrepresented his or her intentions in coming to the country. Prior to September 2017, such actions were at risk of being classified as fraud only if they occurred within the first 30 days of entry.
I can see where they need to have the number of days as cutoff. If someone comes to America and gets married at the airport chapel in Vegas (having made the reservation a week before travel), then I think you can presume they had intent to marry.
Like I said, it sounds iffy.