Those extensions are part of it, but to get the full picture you also need to see the features and limits. E.g. an extension may gate its functionality behind feature flags, so a conformant implementation of the extension may just be a stub that says "feature not supported", or only support a subset of the extensions functionality.
That's annoying. Sounds like vulkan stumbled into exactly the same problem opengl had. There was the core, then a maze of vendor features and extensions differently supported.
The feature maze isn't too bad if you only care about running on modern-ish AMD and Nvidia hardware with their official drivers, most things you'd typically want are implemented in portable extensions supported by both, but god help you if you're writing Vulkan for mobile.
Yes it's convoluted, that's Vulkan in a nutshell.