Yup that's true. Lower is not zero and 13 in a group indoors in a normal sized house for a few hours isn't wise in my opinion.
You’re right, lower isn’t zero. But is this about absolutes? There’s Covid in the village… should I go shopping? Tonight I’ll be in a restaurant, having showed proof of vaccination to get in. Someone, even though vaccinated, might be a carrier. My risk assessment is catching it is highly unlikely, and if I do catch it I’m triple jabbed and shouldn’t become seriously ill.
Short version, I’ll risk assess my activities and, were reasonable, get on with living.