I would assume that most of the code is the way it is because "helping users flag accessibility concerns in real-time" in the about implies that they are play testing games using Chroma on top. Using OBS for this would require insane bitrate and tight latency restraints that do not sound very achievable.

Also, at no point does it look like they are actually recording anything. Just screenshots.

I never mentioned recording or streaming. You can have OBS preview a scene with filters. Plenty of streamers have played games via an OBS preview.