I think you'll find that if you click the "Show My Addresses" button, Address Book will open and you'll find that the greyed-out addresses are identical to those on the "Me" card in your Address Book....
No way to disable the email sender check that I know of. You could use Help->Send Feedback to ask the Omni support group if there's an undisclosed option accessible via the Terminal to disable it, but I suspect there isn't. If you want to be able to send mail from any address, there's the
send-to-omnifocus@omnigroup.com approach, though it doesn't offer all of the functionality that the mail rule approach does.