When Christian remarked that everything his son experienced was "real," including the sideways world experience (created by everyone as an afterlife holding tank), the debate still centers on what is "real."
A simple breakdown of key elements of the Island would help.
Real, unreal or science fiction.
1. Passengers surviving a mid-air mid-ocean plane separation and crash.
Not real. There are many real world examples with the same conclusion: no survivors.
2. Passengers believing they survived a mid air mid-ocean plane separation and crash.
Science fiction. There would have to be some story mythology explanation of how the
passengers could survive a 20,000 foot plunge. TPTB never gave an explanation of how
the characters survived the crash. A sci-fi explanation could include that the characters
"re-materialized" due to the unique EM properties of the Island, like spirits being reincarnated.
3. The Smoke Monster.
Science fiction. There is no basis in current science, technology or nature for an intelligent mass of black smoke that can shape shift into various forms, including dead people.
4. Time Travel.
Science fiction. The turning of the Frozen Donkey Wheel displacing certain characters into different time periods throughout history cannot occur on Earth. The inference is that the wheel was somehow attached to the life force pool, which somehow controlled time and space for the island and its residents, and when turned, major changes would occur. However, these time-space jumps did not create any paradoxes for the time travelers.
5. Electromagnetic Energy.
Real and unreal. EM is a real physical property on Earth. EM can be manipulated to create forces which can be used in working machines, such as bullet train propulsion systems. But the use of massive electromagnetic energy assaulting Desmond in the hatch "implosion" and Desmond's being thrown into the massive electro-arc by Widmore would have killed a normal human being.
For those who accept the sideways world explanation, but somehow grasp to the "real" comment for the Island explanation - - - as being a "real island" on Earth with "live" characters who survived the plane crash, one must review the key elements to accept the objective reasoning that the Island is purely a science fiction realm and not a real island.
If one starts with the premise that the Island is not real, but a magical science fiction realm, then the unreal events on the Island itself become more palatable from a story construction stand point.