Although it seem that alien artifacts do get produced on worlds with ruins...this doesn't get reflected in production totals and they never get made available for export.
Examples from my current campaign:
This colony has a population of 103.3m and never had ruins, as a control example.
This colony has a population of 86.4m, and while ruins were present they have been completely excavated. Note the negative import demand for artifacts and no exportable supply.
This colony has a population of 50.2m, and the ruins present on the planet are still in the process of being recovered. Again, negative import demand, no supply for export.
edit: the size of the ruins seems to determine the amount of phantom ancient artifacts the colony produces: the second colony in my examples had about 1300 installations initially, the third had about 500, and this colony with a population of 40.4m had over 1700 (!) installations before I started digging.
They don't show up in the production, but the significant negative import requirements you see here only show up on worlds with ruins. Clearly something's going on, but ancient artifacts are not being reported properly in the annual production column, and they're not being made available for commerce.