It's funny that you think that they are building their encounters even remotely based on D&D tabletop rules. If they were, you wouldn't fight imps and intellect devourers and demons at level one and two with potentially only a party of two. If anything, the entire game so far seems to be building encounters around a party of six. That's why they're nerfing all the enemies.
Think about it, if I fought three intellect devourers with a party of six level two characters, that would still be a challenge if they implemented the rules correctly. A party of six fighting 3 imps would also not be an easy battle. The whole game is nerfed right now because we are only having four party members. If they want to make this game work even remotely close to D&D rules, they need to implement six party members and stop nerfing the enemies.
But regardless, the concept is still the same. A six-party member group fighting the same enemies as a four party member group will gain less experience and it will thus balance out. They don't have to rebalance the whole game for just that reason.