True, but a poorly optimised game (irrelevant of the hardware) would make for a worse experience. They need to find a balance.
He's used 5gb of the 8gb available ram. He's loading everything into 1 chunk, or something along those lines, which means that you only need to load the entire world once, and it should be fluid. No texture pop in, no loading after the initial load, etc. It shouldn't be a worse experience at all.
If he started "optimizing", it's more likely that it'd make for a worse experience, as he'd need to separate the chunks and condense everything.
Framerates tend to struggle when there's a lot happening on screen at once, obviously. What we might consider "poor optimization", or low framerates for no apparent reason, is almost always due to the game trying to load stuff that's out of your field of view, or if the game fails to unload stuff that you have loaded -- but these things are related to several things; rendering solutions, GPU, CPU power, etc. It's not really related to what he was talking about, since he was simply talking about how he was using the ram.
I think that's how it works, anyways.