He is actually correct. Back in the days when the DAG file was like > 2GB, I was upset that I couldn't mine with my 2GB GPUs. And there was a developer from the eth foundation that basically was working on a miner software where it would split the DAG file and make it possible to mine on 1GB, 2GB. The miner worked in simulation mode at reduced speeds, however it never worked in a real mining situation. Hence the project was dropped. However lately it seems that lolminer has figured out a way to split the DAG file and possible to mine when DAG exceeds 4GB in the coming months. However... the speeds will be much slower AND with every new dag they will slow down and down and down.

The reason why you can do this in video games is because they don't require as fast speeds as mining does. So in a video game the memory is loaded either on the RAM or SSD and later transferred to the GDDR, however it doesn't need to had super fast unlike mining. So as long as gameplay isn't interupted its fine, however for mining it wouldn't work.