On the other hand, that was purely due to his counter-offer being 80%.

Given the amounts we're talking, there's basically no difference between $1500 and $2000 for a big company - the entire process already cost them as much, and continuing said process would keep costing them more especially as it's a clear-cut case of breach of contract they are unlikely to win regardless of how much time they spend on it.

If they didn't believe he had any ground to stand on, they would've declined immediately regardless of how low his counter-offer was. Given that they accepted it immediately, they knew they were in the wrong and would've accepted a repayment of the full amount just as well.