Well, he said it from that point on. See section 5.16 of the rules.

First, the rules can be changed by mutual agreement.
Second, I only gave an example – how long and tedious a battle can be; instead of butterflies, it could be 1 Devil, for example...
And in general, why are you clinging so much to these rules? In my opinion, the main thing is point 6, and everything else can be changed, and even if you violate it, you can inform your opponent – something like, “I did this and that” – it’s unlikely that he will demand strict adherence to the rules...