It's observable. You simply ask people what they have been or are thinking or sensing. If they are able to relate that, then, by the definition, they were conscious about it. Of course they might be lying or misremember; introspection is not intersubjective, and therefore hard to create reproducible experiments around. But that doesn't mean introspective information doesn't exist.
> says nothing about the phenominalogical experience of consciousness in a meaningful way
What kind of experience is observable, except by introspection?
The only observations of experiences that anyone can have are through either one's own introspection, or through what others have related of their introspection.
If you are to say that you have had a "phenomenological experience of consciousness" that you want me to explain, then you are giving an introspective report of a brain process that left an imprint that you can now describe. If the experience was not available to introspection, then you wouldn't be asking, because you wouldn't realise there was an experience at all.