Yes, diet always determine if the immune system function properly. I think good diet might blunt the effect of one variant being better at evading the immune system (but there's no direct evidence for now). So, a poor diet + being hit with G614 variant would probably create a cumulative effect.

Also, I think diet is more important in the early immune response, such as macrophages or antimicrobial peptides (AMPs). Vitamin D is required for the synthesis of antiviral AMPs, for example.

The antibody response is the second line of defense where it may not bind properly to the G614 variant. So, if the early immune response can eliminate the virus, there's probably no need to mount an antibody response.