The basic problem is that “sustainable” aviation fuels, if based on biofuels, would substantially compete with food production. This limits their scale pretty significantly, so they can’t easily scale up to the levels that the airline industry wants, which means that the cost will remain quite high.
The top-level post uses a gift link which may have a view count limit. If it runs out, there is an archived copy of the article
That is a very important question. SAF is a name for a bunch of different fuels produced from sustainable sources. I posted a larger reply to the main article that details some of the input feedstocks which answers your question here.