The Gulden dev reward is not needed to pay the developers because the Florin premine is worth 5 million Euro and only becoming worth more every day. Use the btc to buy Florin.
That premine you are talking about has been sold to (a group of) investors and not anymore in the hands of the XFL dev team. We don't know for what price they have been sold but I do know that the xfl price was around $3 at that time. It seems to be one of the reasons the price has taken off (speculation of who are the investors?)
I bet the group of investors will pay for development, they making a lot of money. It would be a dumb plan to rely on Gulden for development when most people are moving away from Gulden.