What if Jeffrey isn't really an inhuman so much as being 'not human'? They have dropped tons of references to LMDs, we have a 2nd gen LMD in the form of Aida. We all saw in CA:TFA what appeared to be a LMD at the 1st Stark Expo.
You'll note the square build of the 1st synthetic human and how it matches up nicely with Jeffrey's build. Jeffrey's existence could be something classified higher than Level 10, and as such nobody else knows. They dropped lots of SHIELD origin references and most notably Peggy Carter and her founding SHIELD and building the base. I foresee some episodes going back to that time in order to also wrap up the Agent Carter story that got left incomplete. With that, we could get a glimpse of Jeffrey back in the 1950s and looking the exact same. When May punched Jeffrey, it was like hitting solid rock. He didn't move, he didn't feel any pain, he didn't even react. More of a robotic reaction than an inhuman one. I can't see a human or inhuman taking a punch and not reacting at all to it.