Most companies don't even show up to court, because the cost of paying a lawyer is more than you are suing them for in small claims court.
If they did show up to court, they would then have to argue that (a) their terms of service take precedence over the law -- in many juristictions, it is illegal to write a contract preventing someone from suing you for particular conditions.
And they would have to argue that (b) their terms of service are still valid, considering that they aren't honoring their contract in even paying the OP in the first place.
If OP took Upwork to small claims court, it is almost guaranteed that he would get the money owed to him by the clients who paid him that Upwork is currently holding onto illegally.
People mistake them for absolute laws but they can be challenged and many of them are not even enforceable.
Are such agreements enforceable?