Not exactly, as you can be immunodeficient. It is still inaccurate. If lockdown was proved to prevents you from sleeping well and to cause enough nutritional deficiencies (don't have the english word, sorry), yes, maybe it could perturb the endocrine system enough ot "suppress" your immune system.

But i'm fairly sure everybody that was not eating in restaurants every lunch ate better than usual (still too much sugar but...). And fitbit data in the US seems to shows that people are sleeping better during the lockdown[0]. This argument is based on bad logic, bad science, bad everything.

Honestly, i'm not sure if overall, lockdown is better or worst for the health of everyone. Im sure some people will have a lot of positive effects (me at least), some people might suffer more negatives effects, and for other it might balance out.

From the quick search i did with sleep quality and eating habits, it seems that overall health is better. However i have two addiction doctors in my family and anecdotal data seems to suggest that for other people, it was deadly (one of the two was involved in a Covid unit until last week and just started counting deaths).

[0] https://www.fastcompany.com/90499017/fitbit-data-shows-were-...