The code is. What about the hardware? I think in the first version they were using Rasberry Pis (you can see their old posts), but the most recent picture seems like a proprietary device.
From the GitHub README (aka in plain site) "device/ - The firmware which runs on ESP-32 microprocessors, handling message IO, user interaction, and display."
I see them connected to the phone via USB. Not "air gapped" in my book. But you are free to trust them.
Using devices does not mean they're connected to the internet. FROST requires communication between signers, plugging devices together simplifies this communication.
Seems like a huge red flag to me. Can't this be abused? Maybe i'm wrong. Personally i don't need any of this nor i would recommend anyone to use. But it's an interesting project i will give them that.
Study FROST some more. Stephan Livera does a great episode on it.