This argument does not work, because we are not limited by available space in total agricultural output. Just consider the Netherlands: Second largest food exporter despite the US being >200 times larger.

Most of the richer countries/trade unions have a large meat surplus that could be easily shifted to something else, too.

Fish don't care about soil quality or level ground. If anything fish ponds can benefit from height differences because that allows you to flow water through multiple ponds before pumping it back up

Obviously nations do have limited surface area and creating new agricultural areas for them would be to the detriment of forests and "nature"