IIRC the cinematic aspect ratio is 2.39:1. Neither their listed aspect ratio nor their resolution is that.
21:9 - 2.33:1
2560x1080 - 2.37:1
So the only way you would get the correct aspect ratio with that display is with non-square pixels. I am guessing the actual aspect ratio of that display is 2.37:1. I would not recommend buying a display from someone who conflates three different aspect ratios, gimmick name or not.
2.37 (64:27 = 43:33)
As of 2010, TVs have been introduced with this aspect ratio and are marketed as "21:9 cinema displays". This aspect ratio is not recognized by storage and transmission standards.
Ug, you're really missing the mark on that. 2.37:1 is so damned close to (depending who you talk to) the proper 2.35:1 or 2.39:1 AR for movies as not to be notable in difference. You are going to really split hairs over the difference between 2.37:1 and 2.39:1 AR? This would be ideal for black-box removed 1920x800 Blu-ray native resolution content in a HTPC environment. 1920x800 would be proper cinema aspect ratio but for a HTPC 2560x1080 makes a lot more sense combined with functional desktop use.
Simple math:
2560/1080=2.37:1 AR
1920x800=2.4:1 AR
See, not even Blu-ray gets the proper AR for movies right.