I think getting this working took a lot of effort and insight, and I don't mean to discount this effort or insight at all. I couldn't do what these guys did. But what I mean by "toy problem" is it avoids a lot of stuff about the physical world, hardware, laws, economics, etc. that happen when you try to build real things like cars, robots, or helicopters.

In other words, I think it's great people figured out the ideal rocket equation. But somehow it will take a lot of elbow grease (that Elon Musk et al are trying to provide) to make this stuff practical for people who are not enormous space agencies.