this is slightly mis-led, but it is true that 3d conversion sometimes doesnt go as well (and often happens at the last few weeks of production, done by the some of the lowest paid people in the industry)

so there are different levels of "made up".

the VFX companies spend months to make the shots look perfect, and then the 3d conversion company rotoscopes some elements creates close enough 3d geometry to project it onto, shifts it in either direction (sometimes both, to try and maintain the original perspective) and finally re-paints or smartly clone-tools the missing elements that were previously behind other elements.

it's honestly surprising it's done as well as it is considering this has to be done once the movie is absolutely finalized and before it goes to the theater.

Yes, but I assume the “fake 3D” films don't do their CG effects in stereoscopy.

But I mean you can 'make up' new content. That's literally what they do for 'fake 3d'! They 'make up' new background content for areas uncovered by the shift needed to create the parallax, using either machine estimation or manual drawing. How else did people think this worked?

Yeah, the page is a bit disingenuous, and they're all affiliate links.

You need a depth map to compute parallax.

Yes they make one using simple models that roughly fit the geometry on screen. Then transform for parallax. That may reveal hidden areas so they fill them in either using an algorithm from surrounding areas and recent frames or manually.

This is explained on the website, linked from the main page, as the reason why he stopped updating the page.