Yeah, my understanding is that you only run into trouble if you hit all three of (1) a holding out, (2) to carry people or property, (3) for compensation or hire. The problem is that for this purpose, "compensation or hire" includes taking a pro rata share.
So, for most of the "fly around with a friend" situations we're used to, you can accept a pro rata share because you're not holding out. That's what gets flight boards etc in trouble. Even offering to accept your pro rata share is a problem if you're broadcasting your availability to everyone and anyone.
For the Angel Flight scenario, they're holding out, but not accepting any money at all, not even a pro rata share. (The weird IRS fuel thing notwithstanding and operating under its own exemption.)
So, you can hold out / advertise, or you can take some fuel money, but you can't do both. (Also, as a private pilot, even if you do take some fuel money you have to pay your pro rata share no matter what, but we all know that.)