No, not quite.

A butt joint made with a backing block would, as you suggest, provide more
than enough strength compared to a scarf joint; however, it would also
provide a "hard spot" that would not provide a continuous curve when bent,

Yeah, but he's not building a boat. It's just crown moulding and probably
not being bent (although a lot of new homes are very "bent"). Why mess
around with a planer and jig when all you have to do is throw it in a mitre
saw and glue it together. Suit yourself but crown shouldn't need to be that
complicated. Of course it looks like you could use that 8:1 ratio scarf
joint and put two pieces together on the ground and put them up as one
piece. That would be pretty cool. Not practical for me to do at work though.

Kevin