Nobody is LARPing an Eingetragener Verein, they're just being one, and the law requires them to have a board and presidium.

As they got hundreds of votes on the issue, that means tens of thousands of euros in membership contributions. It doesn't seem unreasonable to want to create a legal entity for that, and an e.V. is easier to set up in the beginning (although it can lead to a lot of drama later).