I guess they mean that there is still the gap between molecules and perception. There is no concept for pain and thus no explanation of how Ibuprofen changes the perception of pain. That said, how do you exactly point out that an explanation is not sufficient? It's like describing a hole, you can only point at the borders.
“There is no concept for pain” is only true if you believe that human experiences do not map to the physical world. Otherwise pain is entirely a manifestation of physical chemical interactions. Pain is actually a pretty well understood phenomenon. Perfectly understood? No, but that’s typical of most things.
As for pointing out that an explanation is not sufficient, it is perfectly reasonable to point out the gaps in our knowledge. If those gaps are extremely large, it’s reasonable to call that out as well. It’s not reasonable to claim that the knowledge we do have is meaningless simply because there are still gaps in our knowledge. We don’t understand the origin of the universe, and yet we still manage to fly rockets pretty well. We managed to predict eclipses with perfect accuracy long before modern astronomy existed. The gaps in our knowledge don’t prevent the existing knowledge from being useful or accurate.
