Ha! I think I was just reading your comment over there.

I just tried it with every variation I could think of, no go.

There is a popup message at the top of the Hilton points page saying they've updated and changed payment options, so maybe it just happened to break this process in the last days?
I even tried an end run by attempting to add my Hilton GC as a card in Google Pay - but Pay would not let me add the Hilton GC as a regular credit card (no shocker there), and although it let me add it as a gift card - that doesn't make it available on the points.com Hilton page as a payment method.

Oh well, would have been convenient if it worked!

BTW: When you check the GC status on the Amexgiftcard site, the attempted transaction does appear as a decline, so it is Points.com giving the thumbs down in this process. Which again, does not surprise me, but kind of funny it worked for others even apparently until yesterday if the one FB comment is accurate.