"Submit all showing you are exempt from filing a form I-864". Hmm, now this is a good question, when did you file I-485, there was a period of time back when I-944 was a thing where seeking this exemption was just a checkbox on form I-485 itself, after they stopped requiring I-944 they just started asking for I-864W again.
But if we were to take what the lawyer said at face value, where USCIS did explicitly request an I-864W, print out the RFE notice, fill out and sign form I-864W, attach a note that this is additional submission following submitting the medical alone, and send it out.