In the example in from that what if, they are putting a universe's worth of mass in the volume of the moon, so it would create a super massive singularity. That's not what is happening in here.

If every atom suddenly gained an electron, they would indeed increase in mass. But a hydrogen atoms would gain the most relative mass as it is the lightest atom, and that would only be an increase of 1/1837th of its total mass now, so... not that much. Masses of heavier atoms and the macro level matter made from them would increase in mass even more marginally. It would be a negligible difference, definitely not be enough for a singularity to form from this increase alone unless a star's core were already riding that edge.

So their original determination would still be correct, that molecules would fly apart (atomized) and explode outward into the vacuum of space. Now, maaaaybe if the explosive force were enough to cause atoms to collide in space and at relativistic speeds, tiny singularities might form. But their combined negative charge would be far more powerful than their gravitational pull, and they would decay almost immediately, so... no crunch.

Grain of salt: I love physics, but I'm not a physicist.