The company you contracted for wasn't claiming to be a platform that let you hire your services out to other people, was it? If Upwork or a recruitment agency s

The company you contracted for wasn't claiming to be a platform that let you hire your services out to other people, was it? If Upwork or a recruitment agency simply assigned you to a client at a rate they chose without telling you what it or the project was until after you had accepted and expected you to accept at least 80% of those projects unless you had previously signalled you were unavailable, they would also face issues. For your contract to be legal it would have to pass other tests, but for you it was an objectively much stronger position (your agreed a rate you thought your time was worth when you signed the contract, not whatever your client thought they could hire you out to their next customer for)