If I'm hiring for a real time video streaming platform, I probably don't want to hire a backend dev who has 0 experience in the field and takes 100x thr time, so this seems fine.

The point of the assignment isn't to make it "fair" (in some loose sense), but to filter out candidates.

True to some extent, but you usually don’t want to filter out all candidates who don’t have extensive experience in the exact domain you’re working in, otherwise you’d never hire anyone.

The take homes I created in the past were not so extremely domain specific. They were relatively common problems like "read these JSON files and generate a tree of dependencies between them based on references, where file1.json might contain an item like 'file2.json'".