That article seems rather dubious - I would have thought that Architectural Digest would be above publishing such an article, or at least be more forthcoming on what they are basing their assertion that Fort Belevedere (FB) has been "earmarked" for the Cambridges.
It has been mentioned in previous discussions about Fort Belvedere on this site that a non-royal has a lengthy lease on the property, probably similar to the lease that Andrew has on The Royal Lodge. Galen Weston, the non-royal, passed away in 2021, but it was a given that his lease was going to outlive him, so it stands to reason that one or more of his heirs can occupy Fort Belevedere until the end of the lease. Perhaps his heirs aren't interested in occupying Fort Belvedere, and if that is the case, then presumably they will sell the remaining lease to another party. If William and Kate want to occupy FB then they can buy the lease, but I am not sure if they can afford it.
When royals take over new residences, there are invariably articles about the money involved in acquiring and/or upgrading the property. I am pretty sure if the royals really want a property, they will take their lumps in the media and with the public, but I am not sure that FB is a residence that is desired enough for the Cambridges to endure the negative press.
As I mentioned before, if the Cambridges move to Windsor, it makes the most sense for them to move into the castle. Since Windsor Castle is the largest occupied castle in the world, I am pretty sure that there space in the castle for the Cambridges to occupy without The Queen feeling like she is being encroached upon.