It's the technology used to create it that's out of place, though.
Imagine if we discovered that Leonardo created a working space ship. That'd be a pretty big deal because it's way ahead of the technology that time period should have been able to pull off, right?
It also suggests that they likely had other technology very advanced for their time and possibly even a rudimentary civilization that left no record behind.
Yeah, no. Carving is not some high tech endeavour. If you found some micro-processors that'd be a big deal. A lot of our early tech was easily within reach of human capability, it just required people to build up the knowledge to get to the stage where we are. All it took to get to the iron age was for someone to blow on a forge.
And is it really surprising that humanity has had several shots at civilisation? I don't think we got where we are now from one go. It's likely that pockets of humanity has at several times in our past made progress only to succumb to disaster, war or disease. Maybe some of their knowledge escaped to raise our knowledge base so the next ones to try had a better shot at it. But I seriously doubt it happened in one sitting.