No, CloudCaptain is based on Linux, and tries to provide a minimal, though Linux-based, image.
Unikernels do not use Linux at all: you can always try to minimize Linux but fundamentally it is a monolithic OS and fully specializing with it would require non-negligible engineering.
Instead, unikernels are (typically) based on a modular OS that makes it easier to pick and choose modules for each target application, resulting in images that can be an order of magnitude smaller, boot much faster, etc.
The difficulty with unikernels in the past has been to (1) making them Linux API compatible, (2) making them accessible/easy to use and (3) integrating them with popular tooling ecosystems (e.g., Docker, Kubernetes, Prometheus, etc.)
Unikernels do not use Linux at all: you can always try to minimize Linux but fundamentally it is a monolithic OS and fully specializing with it would require non-negligible engineering.
Instead, unikernels are (typically) based on a modular OS that makes it easier to pick and choose modules for each target application, resulting in images that can be an order of magnitude smaller, boot much faster, etc.
The difficulty with unikernels in the past has been to (1) making them Linux API compatible, (2) making them accessible/easy to use and (3) integrating them with popular tooling ecosystems (e.g., Docker, Kubernetes, Prometheus, etc.)