Yeah, the dude who guards an empty room seems to have missed a few critical social queues, which is probably why he's been assigned to guard an empty room. It's likely that his supervisors are not so pointy-haired that they really think he needs to "stay sharp" and not read books or play on his phone: It's that he sucks and they're trying to get him to quit because they either can't fire him or don't want to deal with the hassle. They apparently do a similar thing to force teachers to quit.
Isn't that all kinda Graeber's point though? That we have a mixture of pressures from regulations, unions, office politics, etc. that result in people being paid to do jobs that don't need to be done? The empty room does not need to be guarded for the museum to be safe, and the receptionist's existence doesn't really need to be justified in order for the company to continue to function (in fact, it might do better otherwise). The managers who allow most bullshit jobs continue to exist are usually acting in their own rational self-interest, and some people benefit from the existence of a bullshit job, but that doesn't make the jobs any less bullshit.