Are they legit?Speaking as a software engineer, there are ways to capture an audio stream as it moves through the system, and I have no reason to doubt that the software works as claimed.
The other question is whether it is legitimate and legal to use such software. The music label would consider it to be piracy. It allows you to make a copy of music that you can continue to use without a Tidal subscription. From Tidal's standpoint, it is a violation of the terms and conditions of using their service (many of those conditions are required by the music labels in their license agreement with Tidal).
You could make the argument that you are operating within the spirit of the law if you intend to delete the music files after you end your Tidal subscription, but then we are getting into a murky area. To be fully compliant you need to be using a device that supports Tidal.