The point you make about the tidally-locked orbit doesn't hold. Tidal forces are there whether or not the object is tidally locked. The tidal forces would be high enough to rip the Moon apart if it's as close to the Earth as in this article.
The Roche limit can be derived for rigid bodies (like the Moon) or liquid bodies. The one for rigid bodies is closer to the central body than the one for liquid bodies.
That's what fused them in the first place but they hold together on their own now. Like an asteroid - it's a big rock, gravity is not the force holding it together.
Unlike the earth which has a liquid core, as far as I know the moon is a single solid chunk of rock. So the Roche limit is not an accurate measure - you need to take into account the tensile strength of rock.
> The point you make about the tidally-locked orbit doesn't hold. Tidal forces are there whether or not the object is tidally locked.
You misunderstood me. In a rotating body the tide would cause the object to flex on every rotation, given enough time it would surely break apart from material fatigue.
But in a tidally locked body the force is applied once (a stretching force) and stays that way, so there is no fatigue - either it's strong enough or it's not.
"Earth’s Roche limit, the distance from the Earth where its tides could break another object apart"
It's a very common error. This limit is for an object "held together by gravity alone"! The moon is not held together by gravity alone - the dust on the surface is, but the rocks in the center are all fused together.
Additionally since they assume the moon will continue to have a synchronized orbit (the same side always faces us) it won't feel the tides in the same way. It would feel some constant stretching, but not the severe flexing caused by rotation.
The closer side would fall "down" and the far side would fall "up". Only the center would be in free fall. I suspect the moon could survive that - but I'm not sure.
If not, it would spread out into rings.
There's other errors too: In a situation like this the Earth would also synchronize it's orbit. A day would speed up and the moon pushed higher until the two matched. This would stop the flood, burn scenario in the article.