You're not misunderstanding. The description is wrong.

"Any set/del after an undo should disallow new undos"

Should actually be:
"Any set/del after an undo should disallow new redos"

In other words, after a set or del there is nothing to redo.

Check that you don't have an extra space in the output string. I had that problem, it's hard to spot.

Just a heads up on this one. Do not modify the heights input parameter.

The random test case was failing for me because I modified heights. I made a copy of heights instead and the test case passed.

(I did the python version)

The english in the description is a little awkward.

I would suggest the following:

"Peter is a teacher in a school. He wants to publish the names of the students who passed a recent exam in alphabetical order."