Sure. Specifically regarding not storing firmware: The TKey does have roughly 6 KB of immutable firmware which is responsible for loading device applications sent by the host. This firmware receives and then hashes the application together with the Unique per Device Secret. The resulting hash is handed to the loaded application as key material, which is now unique per device+application combination. This model doesn't prevent storing applications on the device, but to enable the user to have flexibility to load other apps we would need to change things a bit.

Regarding security: This is a question of measured vs verified boot. Until the TKey, users of USB authenticators had to choose between security and flexibility. Yubikey is not updatable at all, but several USB authenticators have updatable firmware using signed firmware updates. Some of them are open source software, and of those a few are open source hardware in the sense that they provide BOM, schematics and PCB design. We do all of the above, and in addition we provide the HDL / "chip design" / FPGA configuration. In any case, having a USB authenticator which is updatable using a signed update begs the question of who is allowed to sign updates, which then leads to central control and a long tail of interesting use cases not being signed. TKey aims to combine security, openness and flexibility. We accomplish this thanks to unconditional measured boot as a KDF, as opposed to verified boot.

There is more information on our website.