Alright, let's sort out the thoughts.
First, whether it's genuinely a drifting bottle still can't be fully confirmed. But by merely analyzing the material of the bottle, the technological methods already surpass the current Medieval level. So, either a wizard crafted this bottle, or it was made by the legendary Ancient Wizard Civilization. So, in a sense, this can be considered a time-traveling drifting bottle—a Time Capsule.
Moreover, the Drifting Bottle being inside the Giant Squid indicates it was obviously done by human hands, given that squids shouldn't have a penchant for "inserting odd objects into their bodies." This indicates that someone wanted to achieve a certain purpose using this method. However, they failed, otherwise the drifting bottle wouldn't have ended up here.