They do very in width, but with a proportional font that’s true even with ASCII text. What grapheme clusters tells you is how many times you have to press the arrow key/backspace to get to the beginning of the string.
Only if the text editor made some bad assumptions. You're forgetting about non-printable characters, such as the LTR mark. These are not part of grapheme clusters (or are their own grapheme cluster), but the cursor shouldn't probably stop at them.
You know it's been a long time since this conversation but I think, reflecting, it has to do with grapheme clusters not being particularly consistent across operating systems and over time. The article even has an example where one Unicode spec encodes the same 5 USVs and either 1 or 2 graphemes.
