The format is defined less by its length than by its distribution. A short vertical video posted where nobody sees it is not the phenomenon; the phenomenon is the recommendation feed that will show a first-time post to strangers.
That property is what makes short-form viable for people with no audience. Reach is allocated by whether people watch, not by how many already follow.
TikTok, YouTube Shorts, and Instagram Reels are the dominant surfaces. Each weights signals differently, so the same clip can perform unevenly across them.