There is really no legitimate reason why MintJellyToes would not be able to allow Sebastian access to his polinex account, at least not in my eyes.
are you kidding! give a random person on the internet access to your funds on an exchange?
do you give randoms on the street access to your online banking too?
i think its pretty clear no one has claimed those funds as of yet, and MintJelly showed ample proof that he owns those funds.
Sebastian is just turning the blind eye hoping MintJelly gives up and he gets to score the funds.
An exchange account is very different from online banking details. It is trivial to withdraw all funds from an exchange account and an exchange account is going to not be associated with your RL identity, and has little/zero value. So a worse case scenario is that MintJellyToes withdraws any coins that he has deposited on his exchange account then gives Sebastian access to his polinex account so he can confirm the transaction was actually from MintJellyToes.
AFAIK, no one else has claims the funds, but that will not stop someone from attempting to claim the funds in the future. MintJellyToes has not shown proof that he sent the funds in question.
Based on the limited information available, I would say that Sebastian is currently in the right. If you (or MintJellyToes) can provide some additional context to why a refund is being requested and the exact terms of the escrow then this may change (although I somewhat doubt it).
edit: if the coins came from an exchange, then I do not even understand how MintJellyToes expected to get the ICO coins if the ICO had not been botched.