That reason is put only to avoid double accounts. If he joined more than a year ago, it's not a double account to vote here.
I agree that there should be some kind of restriction. But it should be not only based on amount of posts, but in date of registration too.
just because they've been here for a year doesn't mean it's not a double. I could create an account now, keep it dormant for a year and then use it, could still be a double.
Actually if it's been here for a year and doesn't have 300 posts then it's either a very inactive poster or a double account. I'd say while it's very likely it's not a double account, it's more likely that it's a double account if it's been here for a year without 300 posts