It's for the compiled zine, which is going to be available at print-at-cost. You are welcome to submit any kind of submission you want, someone is actually making a short film. It's just if it's not a5 it won't be included in the community zine.
Submitting one in the format you wanted and then trying to edit it to fit within a5 also works. But yeah it's purely for the zine so if you don't care about then no worries.