Interesting assumption but false. The places where fish are located is often pretty dense in fish. That is to say, fish are not evenly distributed. They tend to swim in school of fish, and oceans have vast "desert" areas where the bottom is made of sand and very little else.
The Pacific Garbage Patch is interesting target for cleaning because it has a higher density of plastic compared to other areas. The question is if the density is high. Fishing technique has very little insight to give here beyond technology such as radar and echo sounding, but I am uncertain how effective that would be.
Creating an effective device to clean large swathes of low density ocean is going to require novel designs. For high density areas there are divers, and recreational divers are actually one of the current biggest force in cleaning up water around beaches. The problem is that it does not scale.
[1] The entry is very short on Wikipedia: https://en.wikipedia.org/wiki/Pleuston#Neuston
Saying "we don't know, so we shouldn't do anything" is not productive. We know that all that plastic in the ocean is harmful for some marine life, so we clean it up. You can't postpone all action until you know every possible variable, because you never will. Instead, investigate the neuston, investigate the impact of this cleanup, and re-evaluate after we have more data.
Of course, unless the world's nation change their policies, this will be moot and environmental destruction generally will accelerate given our present politics. But shitting on this particular project hardly seems a useful way to force this absolutely necessary general change.