What's been discussed here over the years, pupmuff, is that while a few folks can and have relayed that they did DVC points for Fort campsites, many like you have said they can't. I don't recall if those discussions were for lengths as long as you were looking for.
That makes me think the number of sites available to use DVC points is limited. EVERY available site may not be able to be "paid" for with DVC points.
Sort of like when auto dealers advertise a brand new car for a ridiculously low price. Yes, they may have ONE unit available and after it sells the rest of the customers coming in asking about it get the upsell sales pitch.
Did you check to see if a Premium was available for booking for your dates if you were paying cash like a regular guest stay?
Bama Ed