ほぼ掛け声だけでしたので素人でも問題なかったのだと思います。

五人で順番並び替えて二回録って、三回重ねたらしいので三十人の掛け声です！(笑)

It was almost entirely just yelling, so I think there was no problem using amateurs.

We were sorted into groups of five and recorded twice, and they apparently repeated this three times, so you had thirty people shouting! :-)