Their whole thing was that Scottish independence was a good thing, but it wouldn't be possible (or even a good idea) because Scotland wouldn't be able to support their economy on their own. Does that response hold water?
As a disclaimer we are U.S. citizens, and I definitely don't know much about Scotland, so feel free to dump whatever information on me that you want
There are countries of similar size and similar resources (or worse) that do well economically. Off the top of my head: Singapore, Norway, Finland, New Zealand and Ireland. So in principle scotland could do well post-independence.
However, to really answer the question you'd need to understand how their economy is currently structured and how independence would effect it and how feasible changing that current structure would be.