Pure speculation on my part, but the fact that the $75/year Amex Hilton Surpass just changed up to give free HH Gold with no spend requirement seems to be a strong indicator of the HH/Amex relationship, and a positive note for folks who are hoping that HH Gold becomes a permanent benefit of the Amex Plat.
The devil's advocate side would point out that perhaps Amex is trying to segregate the benefits, and make the Surpass stand out by offering something the unbranded cards do not. But that pattern does not seem to hold when considering the benefit overlaps between the Plat and SPG, or Plat an DL Reserve.