The ColdCard hardware wallet is indeed considered to be a fully open source hardware wallet.
ColdCard's software is open source because it's publicly available for review, modification, and redistribution. Its hardware design is also open source because the schematics and design files are available for public access. You can find the source of these files at the link below.
Since both the software and hardware are open source, the community can independently verify the security and functionality of the ColdCard wallet, and contribute to its overall development. For that reason, the ColdCard is open source.