Yes, you can travel over the exit if you don't have all the keys. However you can only go through the exit if you have all the keys. As it says directly after the sentence you are quoting: imagine that the exit is a trapdoor

From current description:

You can only travel through doors if you have the correct key. However, you can travel over the exit even if you don't have all the keys

Hi, from the description:

you have to collect all the keys to be able to escape through the exit

I will edit it also to make this clearer.

The grid below has solution, but I'm getting that grid has no solution
.#a.C..
.####..
b#.@...
B#...##
.#c.#$.
....A..

Your solution was: [(3, 2), (3, 3), (3, 4), (2, 4), (2, 3), (2, 2), (3, 2), (4, 2), (5, 2), (5, 1), (5, 0), (4, 0), (3, 0), (2, 0), (3, 0), (4, 0), (5, 0), (5, 1), (5, 2), (4, 2), (4, 3), (3, 3), (3, 4), (3, 5), (4, 5), (5, 5), (5, 4)]
However, the grid was unsolvable.

And it has solution as there is way to get the key 'c' after that unlock door C, pickup key 'a', then unlock key A and then exit door