Oh dear-I have mentioned this a long time ago. I translated this as 'everyone is at home' and I was told by ye computer that it should have been 'at a home'. Well, nobody says 'at a home' (unles we mean an institution-type home) and anyway 'w domu' translates everywhere else as 'at home'. I am baffled.
It's rather "every".
In theory, you could say "Każdy jest w domu", it's accepted, but "wszyscy" really sounds more natural to me. It's like a plural version, only that English doesn't use it much... it's more like "All are home", but then this doesn't sound like a common thing in English to me.