I have thought a while ago to build the Seedsigner but unfortunately, the components that are available in my country are just too expensive so I just hang it for some time while waiting for the prices to come down. Anyway, props to you, this review furtherly explains about what Seedsigner is all about.
I haven't audited the code, but I assume the seed is stored in ram while the device is in use. If the OS uses swap files to store the seed it can retain the information unless it's overwritten with random bits as the device is being powered down.
Their whole OS is indeed fully booted up from the ram
[1]. Even if you do a manual installation, it recommends the user to disable the swap file
[2].
This is how they justify it
Not sure I agree with their explanation of wanting to generate entropy via meatspace.
I found this repo which should explain the reasoning why they decided to go that way:
https://github.com/SeedSigner/independent_custody_guide#creating-secure-private-keys-in-a-trust-minimized-way.
~snip~Though there have been advances in the ability of software to generate unpredictable data, disagreements persist on the theoretical ability of truly random data to arise from organized, logical code created by human beings. (This may go without saying, but it's not a best practice to trust a private key generated by a bitcoin storage device that does not incorporate some kind of user input into the process.)
It turns out that the simplest, easiest, and perhaps best way to capture entropic data is via the randomness inherent in the movements of the physical world that surrounds us. ~snip~