You're absolute right and the process is sometimes referred to as 'tuning'. A living fish is generally looking to hold station and avoids the region closest to the obstacle which draws it forwards towards the obstacle. The process is conducted by the natural elasticity of the fishes tissues but the fish does activate muscles in reaction to perturbations in the flow. So, to conclude, I think that the fish is mainly actively putting effort to first find the 'sweet spot' and then keep itself there, the actually act of 'tuning' is typically considered a passive process and can be seen exhibited in any well-positioned flexible object e.g. a ribbon or string placed in a flow will oscillate at the vortex-shedding frequency. So in terms of the question they ask, it is relevant because the fish needs to be able to detect small perturbations in the approaching flow in order to react to them and maintain its position, the goal being to minimise active muscular contractions and maximise efficiency.
Thanks for the detailed explanation. So then the fish seems to actively do better than 'start from scratch' when it falls out of the sweet spot which is the mystery of how.
No problem. As displayed in the video of the dead fish, although initially well-placed and enjoying the neutral flow region/sweet spot, perturbations eventually force it forwards into the obstacle. The live fish has to be able to detect those perturbations effectively enough to avoid similar situations. If not, the fish would have to actively swim back into the sweet spot to begin enjoying the benefits again, as you say 'start from scratch'.
