That's a weird definition. If you divide a large file in chunks of different sizes and you are sharing even 1% of them you are already seeding. And the point of popcorn time is that people are participating the p2p overlay much longer than with other p2p applications, thus seeding longer too.
I understand leechers as those who, over time, mostly download rather than upload.
That is, by definition, a leech. [1]
Seed: A downloader that obtains 100% of the data becomes a seed.
And here in this case, a while after someone becomes a seed the file gets deleted permanently.
[1] http://en.wikipedia.org/wiki/Glossary_of_BitTorrent_terms