Do they say, "Well, considering the rate at which human art has developed, Venus of Willendorf is so primitive-looking that we place it at about 30,000 BC"? I guess this would involve sociological assumptions.
Do they use sociological assumptions? Assumptions about theories of art?
They couldn't, really.
For one thing, artistic styles very often decline from their peak. The third or fourth generation of potters (for example) making a particular style of pot will start producing it in a stereotyped, stylized, and lazy way, they'll cut corners, they'll substitute inferior materials, they'll lose the original elan and be motivated only by the fact that there's a market for their wares.
So this would make it hard to look at two pieces and say whether one is a primitive approach to the other or a decadent regress from it. Once you know what the one or the other looks like by applying other dating methods, then you can use artistic style as a key to dating, but you can't just establish an a priori rule that good art drives out bad, 'cos it doesn't.
Here's a coin of Antialcidas (ruled 145-135 B.C.)
And here's one of Philip III Arrhidaios, two centuries earlier.
Now the interesting thing is that the designs on the reverses of both coins depict exactly the same thing. It's the same guy sitting in the same chair. But what a decline in artistic achievement!