GL_ARB_framebuffer_object spec includes GL_EXT_framebuffer_multisample
No it does not. It include GL_MAX_SAMPLES. It includes the same functionality, but that doesn’t mean that it includes the extension itself.
that means it is incorrect for the driver to generate the error right, since they are claiming to support multisampling?
Yes and no. In that order.
The driver should not emit GL_INVALID_ENUM if ARB_framebuffer_object is supported and you use glGetIntegerv with GL_MAX_SAMPLES. However, that does not mean that multisampling is supported. The extension is very clear that GL_MAX_SAMPLES can be zero, in which case multisampling is not supported. The infrastructure for multisampling exists, but that doesn’t mean you can use it.