I think it just means they want longer individual clips, rather than people making longer clips just be editing a bunch of ones together. Unless it's a really unique clip, or there's a very specific reason something is so short... I don't see much point in creating anything under 10 seconds. I remember the first time I uploaded two clips edited together in one sequence at VideoHive and it was rejected... saying I should split them into two clips and upload separately or together as a pack... so I haven't done that since. The rules may very well be different elsewhere though.