I don't get how is this decentralized.

If you were running this on a dApp, on ethereum blockchain for example, it could be decentralized.

But as you are doing, it doesn't look decentralized because you can just remove the entire voting pool from the centralized website or from the centralized App.

A dApp is decentralized because it cannot be stopped. It runs in a world computer, a computer which no one owns. It is an dApp which is owned by no one.