The distinction escapes me. The test video above was indeed 16:9 originally, made 9:16 when uploaded. It's square & rectangular due to the A/R conversion. There appears to be nothing in your link that describes another process.
Another test was shot 9:16 by a camera, and it succeeded as well. Both are in the "shorts" path of the YT URL and listed under that tab in YT studio.
Videopad offers dozens of options for video export aspect ratio.
Before being taken down, these two shorts had several hundred hits on YouTube, and looked fine.