That's not the way it would work in the attack I'm describing. The malicious miner would still let others mine, it would just alternate which chain they are mining on to help the chain that fell behind catch back up. So instead of splitting 25.5% on two chains, it would be all 51% on chain A half the time and all 51% on chainB half the time. The disruptive part is that there would be switching back and forth all the time, and the two chains could contain very conflicting data.
You know what happens when you do work for half the time? You do half the work. So, having 51% of the hashpower
half the time means you effectively have only 25.5% of the hashpower.
Also, about letting others mine... Well, you can't just stop them from working (unless you seize their equipment). This has nothing to do with it.