Part of it is likely that if they trained the soldiers in the plan beforehand, they risked leaks,

Now, we know there were leaks, and warnings to Netanyahu that he ignored, but Hamas likely didn't know that he would straight up ignore the threat.

There's also the issue of compartmentalization. Hamas operates in cells, because that makes them harder to kill in air strikes. They're spread out, partially disorganized, and try to keep communication between them to paper or in person. All to keep Israel from spying on them.

It's not a stretch to think that the foot soldiers were handed these papers half an hour before they were set to go. It's a sloppy way to run an op, but the op was mass murder and kidnapping, so I'm okay with them being less efficient than they could have been.