That isn't the assumption. Modern archaeologists usually assume that ancient people were as intelligent as we are today, or even more so.
What's not assumed is that they had the same thought patterns. People don't derive ideas uniformly from the space of all possible ideas, they tend to think within the constraints and realities of past experiences. If you build a house, it's going to be similar to houses you've seen before. If you paint a painting, it's going to be a painting rather than some other means of expressing yourself with colored pigments.
In other words, ideas are subject to the same kinds of path dependence that technology is. When we see something that's severely anachronistic (outside of it's "normal" place in time), the initial priors are that things like the dating are wrong rather than ab initio invention of a whole suite of different ideas that just happened to be preserved for us.
What's not assumed is that they had the same thought patterns. People don't derive ideas uniformly from the space of all possible ideas, they tend to think within the constraints and realities of past experiences. If you build a house, it's going to be similar to houses you've seen before. If you paint a painting, it's going to be a painting rather than some other means of expressing yourself with colored pigments.
In other words, ideas are subject to the same kinds of path dependence that technology is. When we see something that's severely anachronistic (outside of it's "normal" place in time), the initial priors are that things like the dating are wrong rather than ab initio invention of a whole suite of different ideas that just happened to be preserved for us.