I don't think you two quite understand what a lurker is. A lurker is someone who never posts. Not "they used to post a bit but now they don't really", but just someone who joins and either never posts, or only posts a small handful of times. Most notably, lurkers tend to only come out during contests around here. This means potentially being a member for years while only posting maybe 20 some odd times, if not less, always during Tempmas competitions or otherwise. They're here for the information and the occasional free stuff. They otherwise don't feel a need to add their voice to any conversations on the site at any point. On boards without competitions, lurkers are just those with accounts who exist. They literally never post for any reason.
After a certain point, you can lurk, but you are not really a lurker once you've made yourself known to the community in regular discussion. You're just a less active member.
And really, there is no way to fairly stop people who purely lurk from competing in skill based competitions. For one, it wouldn't particularly be fair that just because they don't feel the need to post that they be disqualified, especially if it's something like this where someone is skilled and wants to contribute their work. So that's a negative on using post count. If you try to discriminate based on account age, you then have to figure out where the line is. In that scenario, you then risk excluding new contributing members who don't meet the age requirement, regardless of where the line is drawn, and further risk pushing people away from the community who feel excluded right away as a new member.
In the long run, it's easier just to include everybody to get the widest range of entries possible. Even if they lurk, if it's an art based competition like this, it isn't as if just tossing a post in will potentially get a win. If they're willing to work at the competition, they should be just as eligible as anybody else, regardless of how much or how often they choose to post.
Of course, age based restriction does work sometimes, like with a raffle type competition. In that scenario, it keeps people from potentially making multiple accounts exclusively to enter.