On the upside, they don't give you an unlimited amount of time. If they did, then you'd be wondering if you spent enough time on it, splitting hairs on every line of code.

On the downside, this takes 0 of their time and 90 mins of your time. That would be fine if they seriously considered your application, but it's a bit of a red flag because it requires 0 investment in their part. Maybe they are always taking applications, but not even hiring. That would be the smart/asshole thing to do a a business because you always have a large pool of well-vetted candidates to choose from when the need arises.

I'm not an expert, but this is just where my mind goes thinking of it.