No, no, NO! He was SITTING at his desk! Did someone have to pick him up and plonk him on his chair? Of course they didn't. So he wasn't sat at all; he did it all by himself. He was sitting at his desk. As the meercat would say - simples.
Am I the only person who hates this particular grammatical abuse, which seems to be here to stay? Or am I a pedant?