You've missed an important but subtle point. The article points out that the reason this was overlooked is not because Rails isn't profiled but because this behaviour is happening outside the standard rails profiling scope. That is a) why it's not a "relatively-straightforward bug fix", b) why it hadn't been caught already and c) why it is interesting. Actually I take that back, it is a relatively-straightforward bug fix. As with all the best bugs it's finding it that wasn't straightforward.
Properly profiling the full execution path should be an entirely obvious step to take.
Properly profiling the full execution path should be an entirely obvious step to take.