No what it sounds like to me is the company you work for wont sponsor you for 3 years. And they are quite within their rights to do that unless you have something in your contract that says otherwise?
If they agree to sponsor you you wouldnt need to do the skills assessment and can get an age exemption. But not the case if applying independently 58 if pretty much a bar to independent PR
. I would be pushing your manager as to why they wont sponsor because if they wont it is likely you'll be heading back to the US.