I'm so glad they did this, I don't think it'd have been good for kotsume to go back to poseidon and his terrible senpais.
In the end kotsume is someone who needed more flexible thinking and patience to teach, he did so so well learning and sometimes you can't get things you wanted back, but as in this case, you realise something else was even better.
I understand poseidon explaining why he couldn't accept him back too but... Kinda kicking the little guy while he's down to make him feel even more guilty about the spilled wine. I do agree with others that it seems a bit weird to just be explaining this now.