As long as this exists in the manga...
And in the anime adaptation (which cut two other references to the pregnancy this half of the War for Paradis arc)...
Then I fully refuse to believe that the farmer (or Eren) is meant to be the real father.
Yelena and Zeke aren’t idiots. Even though this was not part of the original euthanasia plan, there must be something about the child that would give it a special ability to do what they say it will be able to do that will ensure the island’s safety until all the Eldians die off.
There’s a completely logical explanation for it. Jean may be perplexed by such a stupid sounding plan, but there are many secrets being harbored in this arc.
And yes, I’m going to be a total pain in the ass about this until we get proper answers.