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.
in the past there were problems also with old accounts that turned out to be double
that's how this rule was created...
i do guess it may be inconvenient but it's here for a reason