Was it possible for them to have avoided getting caught? I think so. The reason why they're caught is more of their own faults rather than the capabilities of the FBI to track them down. Of course it is very hard to be extremely careful every step of the way. It might even be next to impossible to avoid committing an act of negligence or carelessness forever. It is hard not to commit a mistake and leave a trace. But it could be done.
If they are not been suspected of suspicious activities leading to the exchanges reports, they are still free till now. But yes, they leave such traces that could help the FBI to detect the couple's fraud activities. Honestly, they are not too smart to hide their doings but they are too bold enough to leave such a trace.
Even we think it was a success story for the FBI in hunting criminals like them, keeping anonymous won't easily give them the idea of who are they. But with their simple mistake could somewhat give a hint to the authorities and that is the reason why they end up getting caught.
On an article I read that they were very careful in redeeming the bitcoins to cash in one way or another. They tried to buy some gift cards and tried on another website which was then accessed by FBI and got their data according to the law enforcement. I think that all happened because of the efforts of the FBI who not let these hackers to escape the money of people.