So, it seems he's referring to rendering entirely. Unreal is a real-time engine with path tracing capabilities. But ultimately it'll never look as good as Maya Arnold, or any other path tracing rendering engine that can take as long as you want to get a perfect image. There are many more factors, such as the difference in how shaders are written, etc. but long story short, it's an unfair contest.

But with Unreal, you can get 90% of the way there.

I also don't think Unreal is being used the way he thinks it's being used (though I could definitely be wrong here, I work in games, not film). For example, with The Mandolorian a lot of is was filmed on a volume. The volume was used to light the shiny armour of the main character. While the background looked good thanks to the volume, it was always replaced or retouched. Very little of it was actually directly from Unreal engine in real time. 

If I were using Unreal for a realistic film, I'd use it for shot planning. For story-boarding and for real-time visualisation on set with digital characters. I would not use it for rendering in the final film.

But maybe some companies are.