The word fumble means to use one's hands clumsily. However, if JB said Rashid fumbled in the football context, it would not be reasonable to argue Rashid was a linebacker that failed to prevent a touchdown because he clumsily used his hands during an attempted tackle. It would be clear that Rashid had possession of the ball and dropped it before he was down.
For the same reason as above, if Rashid focused the tank in the LoL context the only reasonable interpretation is that Rashid and DR were on the same team and Rashid attacked the wrong target.
The WOJ said we got the pieces we need in Cold Days. In CD the barges were the tank (the big obvious target that wants to be the target) and Maeve/Lily were the carries (the main offensive threat). Thus, we can conclude Rashid was a defender in a similar attack and focused on the barges.
This also fits with JB's clarification that it is a little more complicated than a positive-negative situation. That's because the barges weren't just a tank, they were also a legitimate offensive threat in their own right, just a lesser one than Maeve. Thus, one can't definitively say that Rashid made a mistake by focusing the barges, whereas focusing the tank in a team fight is always a clear cut negative.
With regard to the OP, it's possible Cowl was doing something else while Harry went after the red king, but calling the red Kinga tank would imply teamwork and I think it more likely that the reds were being used by, rather than working with Cowl.