@Xinqun The second step turns your floorless walled room (step 1) into a floorless non-walled room (I made that a little more clear now I hope). The three-block add-on might is definitely not the main goal of that and might not be necessary later on, when the bug is fixed. However, it will never cause issues per se, since the staircase will always get rid of those tiles again and the method does not rely on those tiles to adhere the shape. (PS: Obviously, if you use a twisted staircase, the blocks should be positioned differently.)
As far as the 'taking advantage of a bug' side of this -> I looked around for you a bit and found this by using some more specific search terms: https://forums.thesims.com/en_us/discussion/849804/upstairs-deleting-floor-tiles-for-stair-landing-solution
This method (but with decks back in the day), was apparently already being used in 2015 and still works now, > 5y later :)