This is a policy of your union hall? How did they come up with the "we won't let you back in until Oct. 1" date?

Yes, you can be terminated while on disability, once your protections under state and federal law expire - when applicable, usually 12 weeks total. Some states may extend that, but you disregarded the instruction to identify your state; but you were out six weeks so I don't see it happening. You could also check your union contract, assuming you were a union employee of the DWP and not just taking assignments through the union hall.

If you believe your foreman wrote something about you that is untrue, you should be able to address that through your employer's policies, your union contract, or both. Talk to your union steward.