Update… The instant Marriott Gold status offer does appear to work.
I should add that I fully expect Marriott to eventually audit the signups with this offer. So unclear how long the status will last for those not really eligible. Although these audits are rarely perfect, after all folks sign up with different addresses for different accounts (eg business, home)… data matches aren’t perfect… names are written slightly differently (with middle name, first name as nick name, etc). So some will probably sneak through for the full duration of the offer.
Those looking to boost their odds of keeping status might want to use a non-US address, since the Flying Blue program isn’t usually open to US residents and Marriott presumbly knows that.