That statement is simply incorrect. "Private relief" bills have been common since the 1st Congress and usually for much more trivial "individual legal" matters than life or death. They are usually under the media radar, but they happen all the time. Just because you haven't heard of them does not make them extra Constitutional. There is nothing in the Constitution that restricts Congress from doing this.
BTW. Even Scallia wouldn't have a problem with what Congress did. They are not overstepping their authority.
You are quite right. And Congress doesn't exist simply to implement federal court orders and raise taxes and redistribute wealth. This is the mindset that we've allowed to set in, and we must resist it. We are not a government by judiciary, but we're getting there. The framers never supported such power for this unelected branch. This history of our founding and the framers' efforts are crystal clear on this.
I believe it works the other way; I think the Constitution authorizes Congress to do certain things as opposed to restricting them from what they can't do. In other words, if the authorization isn't spelled out in the Constitution, Congress can't do it. They cannot just do anything they think of merely because those things aren't restricted.
Otherwise, their power would be pretty much unlimited.
OTOH, the situation is the opposite for us citizens, we can do anything that is not restricted. Because, again, if we could only do whatever Congress, in it's wisdom, dreamed up to put on a list of permitted actions by citizens, our freedom would be damned restricted.
Instead, government power is pretty damned restricted. Or is supposed to be.
However, it does look to me as if the 14th would allow Congress to become involved in this case by taking the position that Terri has not received due process.