Perhaps at the time when he wanted to exchange his bitcoin, eXch didn't have any USDT in their reserves or not the amount he wanted. It has happened to me in the past. Alternatively, he could have swapped his bitcoin for DAI or USDC. If stablecoins weren't available at the time, he got the next-best thing, ether. With ether, he can buy the stablecoin he wants at Uniswap, which doesn't support bitcoin, only bitcoin-wrapped tokens.