Is this totally true? Vulkan supposedly derives from AMD’s Mantle API. Yes Vulkan and OpenGL have extensions in between major releases, but is that worse?

Yes, coding against extensions means multiple code paths, hardly any different than using multiple APIs when the semantics from multiple vendors don't match 1:1.

https://vulkan.gpuinfo.org/listextensions.php

https://opengl.gpuinfo.org/listextensions.php

At least with Vulkan they learned why proprietary APIs have profiles.

DirectX isn't much better with regard to the extension spaghetti. At the very least when you see a OpenGL or Vulkan version you can be assured that w/e that version includes is supported. With DX you can have something claim DX12 support but in reality it only actually supports some variation of DX10 or 11 through the feature level nonsense.

It is such a nonsense that Khronos finally learned its value, and Vulkan now has profiles.

OpenGL has profiles as well. The Core and Compatibility profile.

Didn't know Vulkan had them too. TIL.