Because he had not done anything illegal, he has not been arrested. But when somebody writes or says something that sounds threatening or menacing, most states allow ‘detention’ for psychiatric evaluation, for up to but not more than two weeks. If they are not severely mentally ill, which is obvious, they are then released.
And this happens a lot. Potential suicides, public breakdowns that are unresponsive, even disorientation and confusion (often by elderly without identification), and yes, what are interpreted as unfocused threats. Many are called in by family members.
The police are not too gleeful about doing this, because most short term psychiatric facilities are booked solid with “two weekers”. And they cannot give serious drugs to most of them.
Virginia has fairly strict laws on his behalf and if they are not followed, a bunch of state inspectors are going to come down on that Hospital like a ton bricks. They have until Monday morning to give him a hearing or he walks. You cannot do back to back 48 hour holds in Virginia, but you can hold over the weekend. They could arrest him again once he gets out, but Chesterfield County will have to charge him with something and hold him in the regional jail.