The site seems like a test for pre-orders/building a mailing list and maybe gathering feedback so here it goes...

Nostr keys are an inherently hot thing, since you're using them to sign/encrypt/decrypt online communications regularly... a remote signer needs to be online

Bitcoin cold storage is the opposite, you're using it rarely, a cold device that's connected defeat the purpose

Maybe this could hold descriptors and be part of a multisig setup, but then it probably shouldn't position itself as cold storage... not sure what the right nomenclature would be though, standalone signer?