This is all solved

All solved all solved all solved all solved

It all comes down to does it say replace rotted decking? If it does, they didn’t adhere to the agreement. If the decking is as bad as you believe, I’m going to say the truss system probably is rotted in some areas. A 7/16 piece of OSB weighs about 30#.

That is true.
What does the contract say?

They thought all the wood was being replaced
Instead of just installing all new on top.

Does the contract say “replace” or does it say “install”?

Do you have code enforcement or building inspectors? If so, it is illegal to put deck on rotten rafters.

Sounds like you are bound by whatever the written contract says.