What is a wormhole?
It's a portal that punctures right through the space-time fabric and connects two places in the universe.
Which means, a wormhole always has two openings.
If you can enter from one side and exit from the other, you can also enter from the other side and exit from the first one too.
Jareth wasn't the only one who could enter the portal from this side; the opposite was also possible.
In simple words, Jareth's wormhole was never a one-sided portal; if there was any being on the other side of the portal, they could also enter it and come inside the 'Sea of Endless Miasma.'
When the massive secret artifact of Jareth was blown apart by the miasmic spear, Zion's armor protected everyone, and most of the residual blow was suffered by Jareth alone.
Mark and others were not blown that far, and they were also able to maintain their consciousness.
In that moment of absolute despair, they saw something that caused them to widen their eyes in absolute shock.
