Knowing that a sound means "I want to mate", or "there is food here", or "where is food", is not sufficient.

It would have to be like a mother telling a calf "This is where I met your dad" or a whale telling other whales, "WE will come back here again next year" or "Where is Wally" and another whale saying "Wally is dead".

This will also resolve yes if we are able to translate and understand what whales are saying, and can repeat things and monitor responses to varify accuracy, but if it turns out whales actually do NOT have complex language and are actually really stupid but make a lot of weird noises for other reasons (like communicating with specific whales or something but only using a few words/phrases in different ways). I would consider that "understanding complex inter-whale discussions". They would just not be as complex as we expect.