I mean, disease has played an enormous role in many wars. Even being a little sick can significantly reduce the effectiveness of anyone in a combat zone, and people under stress are far more vulnerable to being sick. Disease has killed far more soldiers throughout history than bullets or swords ever have.

It can take a lot out of a person to even fight off a cold, and a person in a siege situation usually doesn't have that much to give. Even if COVID isn't a big deal, you don't want an outbreak of ANY disease in a city that is under siege.

You can accuse them of having a flawed understanding of the threat of COVID or the vaccine, but you can't deny that it's a basic historical lesson of serious combat that you make the health of your soldiers a priority, because disease doesn't care if your soldiers are behind cover or wearing body armor.