Siduri here is Gilgamesh's secretary, one of the few people who stuck with him even after he left for years in his quest after Enkidu died.
At some point, Gudao and Mashu have a chat with Siduri, where they tell her about how raised hands and the white flag are signs of surrender.
Fast forward a dozen chapters or so, when Tiamat has emerged and her Chaos Tide is warping everybody it absorbs into Lahmus, creepy twisted creatures that attack everybody. Here's how they look.
You meet and fight a certain Lahmu that refuses to attack you, only doing a curious gesture like it's waving its appendages. You can't avoid this fight, so you gotta attack it until it runs away.
Ishtar and Gudao figure out what's going on, but decide to not tell Mashu.
It should also be mentioned that the white flag comment was just a passing joke at the time. It still is since we never see here actually raise a white flag or anything but this image is harsher in that context.
Also regarding that Lahmu that never attacks, it managed to rescue a dying Enkidu before it succumbed to the wounds you dealt it. It never introduces itself but it still manages to tell Enkidu 'thank you' on behalf of Uruk for being their king's friend.
Some context, this took place long after Gilgamesh returned after losing the elixer of immortality in his legend so Enkidu had already died before. The one in Babylon chapter is a different Enkidu created by Tiamat and was an enemy who belittled humans and led the monsters against them. He was betrayed and attacked by the Lahmu so that his heart (I think a grail) could be used by Tiamat.
My beloved, friend, En,kidu. We, the people of Uruk, have not forgotten, our gratitude, to you. You, gave to our, proud, lonely king, life. You showed, him the path to, becoming a great ruler. There was not, a single person who, did not mourn, your death. There is not, a single person who, has forgotten your passing.