I interpreted the claim as being under non-ideal conditions (which is fair, frankly—it's well-known that the visual and sound quality is better at the beginning of a run than at the end, and film quality doesn't matter if your local theater doesn't ensure it's preserved as best it can be).
Plus, I saw a film viewing of Sinners this past weekend (quite a fun movie, highly recommend it) and some visual artifacts were very noticeable—it was regular enough I figured there was a slice of the film roll that got damaged somehow.
• “The highest resolution that the expert assessors could discern in the highest performing movie theater was about 875 L/PH.
• “The horizontal resolution averaged over the six multi-burst groups measured on the screens of the six selected movie theaters was about 715 L/PH.”
L/PH = lines per picture height, i.e. vertical resolution
This was for 35 mm film.
Also, well-mastered DVDs with anamorphic widescreen can look astonishingly good on an output device that doesn’t interpolate its lines.
The point is that even 1080p TV is an improvement in resolution over what you used to see in cinemas with 35 mm prints.