Ah, I was finally able to figure out the "joke," as I only got the "flat" earlier as I didn't know "huhu." I had just searched for "eulenwitz," and this thing popped up a million times. I knew "uhu" but I was too tired yet to notice the extra H to figure it was a pun. I knew one of you would lead me to the answer though!
For everyone not in on it:
Was sitzt auf einem Baum und winkt? — Ein "Huhu“!
What is sitting on a tree and waving? – A “Huhu”!
Explanation: in German, “huhu” is a greeting, while “Uhu” signifies an “owl”
They're packaging is pretty distinctive as it is, but I'm a little sad they named it after such a cool animal, but then did not seem to have an owl related logo or anything other than the Adventure Glue bottle, which I don't think exists anymore.
What is sitting in the tree and waving (hello/farewell)?
A Huhu
That's not only a play on words. Uhu is the German word for GHO. By adding the H it turns into an interjection and word that's used when people call out in greeting/to get another person's attention. At the same time it's how Germans imitate a GHO's hoot - huhu.