I would like to see the actual report from the study before assuming that those fish were sizing up the others ONLY by observing fights. There's no mention in that news report of aCONTROL group; of course, that's because they're just looking for the "exciting" parts.
In this instance, you need a group of fish that did NOT observe the fighting, and then record which fish THEY go after. If they go for the same fish without having observed the fights, then there is another factor involved, and the logical inference they are claiming is incorrect. However, if the control group reacts differently than the experimental group, THEN you may infer that they observed the differences during the fights and made a logical choice.
Of course, if 2 fish fight, the most likely winner is the one that has a physical advantage, size, or speed, or both. The 'bystanders' would be able to judge this, and it may just be that the biggest fish won the fight, and the bystanders chose to take on the smaller fish, which by no co-incidence was also the loser.
Also, if a fish has recently lost a fight, it will be acting less aggresive, and so any other males in the area would be likely to single him out as vulnerable regardless of if they have seen him fight and lose.
In addition, most animals choose to display and judge their opponents before engaging. This prevents uneccessary fights between uneven matched opponents.
None the less, its an interesting report.