Fortunately, a video does need to have a vertical screen resolution to be counted as a short. So landscape/widescreen videos don't seem to be affected there.

But this can definitely trip people up, especially now the maximum length of a YouTube short is 3 minutes instead of 1. If you recorded a 3 minute video on a phone (or other random vertical screen device like a Game Boy/DS/3DS), YouTube will classify it as a short and there's basically nothing you can do about it.