Yes, moderators are allowed to be inactive. Being a moderator is an elected but volunteer activity, and moderators are free to come and go as they please. Sometimes life happens; Projects come up in work, other obligations take priority, or perhaps you simply lose interest in the responsibility.
At the same time, it is incumbent on all of us to make sure the work of the community is being done. To that end, we installed the 3rd runner up to help with the workload.
If Dr. Chapman returns, he is welcome and encouraged to jump right back in where he left off. His moderatorship will be here waiting for him. But if significant time passes (a year or so?) where he remains MIA, it may make administrative sense to disable the moderator abilities on the idle account.