Eric has a point.
Even something extraordinarily small, like the degree of spin a photon has, could have retardedly cataclysmic effects on an ID traveler.
A subtle change in spin means the difference between a cell having enough energy to live, and not meeting those requirements, because more energy is needed to metabolize the food now.
Case in point: a photosynthetic plant.
In our world, chlorophyll does a crazy quantum dance using entanglement to get the energy of multiple photons to work in concert, so that enough energy is directed to the active site to cleave a co2 molecule, and bind it with water.
If the photons it is absorbing have a different spin value, this dance fails. The plant can't make sugar. It dies. The light is toxic.
Likewise, the energy needed for chemical reactions would also change from the slight poke at the standard model. Suddenly your body wouldn't be folding protiens properly. All sorts of shit.
Outwardly, the universe would look exacty like the one you were from, but your biology just wouldn't be able to survive there.
Rather, you need a universe with *identical* physics. This means a different possible quantum future universe. (A universe with a shared past. The "same" universe, different history.) There are an infinite number of these for any given universe's genesis.
We can assume that zannie and his buds are jumping from universes with identical physics, because they have the same big bang in common. The difference is just the fallout of chaos theory on how that universe's matter clumped. His universe produced dragons, and they made a civilization. The one we are in now didn't. They are, strictly speaking, the same universe, but with different histories.