Doesn’t change the fact that banning people from running for election is inherently undemocratic.

Something being democratic is not the only criterion, because you wouldn't want your neighbors to vote in favor of collectively owning you as a slave, even if your vote against gets counted.

It's just one safety measure - if a politician still would win an election after 8 years (life changes entirely in only 1 year), for example, that's likely for wrong reasons. Like using administrative resource, pro-government mass media, crooked elites etc.