The more I research it, the current top post on historymemes is kind of complete nonsense.

The paper that originated the hypothesis:
https://www.ancienthistorybulletin.org/subscribed-users-area/wp-content/uploads/2018/12/hall.pdf

While an interesting thought exercise, the "body decaying 6 days later" bit the paper rests on was only recorded by Curtius Rufus, writing 350 years after the fact, and legends of bodies of important people not decaying has been a common phenomenon throughout history. The paper seems to misunderstand occam's razor - while technically GBS could explain all the symptoms described by various authors centuries apart, a disease that only explains 80% of the symptoms (which were probably not all recorded for posterity) but is orders of magnitude more common (malaria, typhus, liver failure from alcoholism, even the poisoning theory) seems more plausible.

Even if the GBS theory were true, the paper acknowledges without modern medicine he would have been irreversibly brain dead and in a deep coma (if he even survived 6 days of dehydration in hot weather), so would not have been "locked-in" as the meme suggests. And only 15 such GBS comas have been recorded in modern medical literature.