I tried the same query on a testing machine, it was the only query reported by sp_whoisactive, with the same 'runnable' status, but there was nothing else 'running'.

I always thought there should be some 'runnable' queueing mechanism, with queries moving into the 'running' group, but seeing just one 'runnable' query (consuming a lot of CPU while "waiting", but no reads/writes) and no 'running' ones is making me wondering if I understood everything properly...