He didnt get phished
There is no other way to lose your Steam account, besides a comprimised account on another program or website with the same password, or having a backdoor on your PC.
Although, it is possible to spoof your SteamID to be like another person's - allowing you to get them banned. Though, this only affects the ingame details, so it's not as if they are using your account, though could get your account banned from certain servers (And theoretically by VAC)