What does "shortlisting for this post is still in progress..." mean in the context of a UK job application?
The message means what it says. The shortlist hasn’t been decided yet. Whoever wrote you the message does not have more information to provide you at this time, or if they do, then they are not at liberty to provide it.

Any additional layers of interpretation that you choose to add to this message, such as inferring that you are longlisted, you add at your own risk. Certainly no one here can tell you whether such an interpretation is valid or not.

Good luck!