Contrary to the video, I started by duplicating the bottom edge where the stairs should start, immediately split it into its own object, and started by extruding up and in by 0.1 (i.e., 10 stairs per big block). So, no need for snapping …
After that, it was just some creative playfulness (see last figure), but because of that, the relative offset was a bit more annoying, and I found the absolute offset such that, whatever I do that changes the overall dimensions, the array itself does what it has to do to create my 80 steps.
At the end, I merged vertices (applying the array creates vertices that coincide), and on the last step I fixed the geometry: I made a “spare out” (no idea what the English word is) such that the repeating steps do not create hidden geometry, but for the last step this hole needed to be filled as no other step is built upon it. What I realize now: there is a small square on the side which is being repeated and which should not be there (only the first step can actually have this because it slightly peeks out of the railing)
