Using past simple in main clause with since - Is it correct?

I read an article on a website and found a sentence that I need to know whether it is correct or not.

Can we use simple past tense like in the following sentence (in bold). Is it correct? Don’t they have to use present perfect tense if they use "since" because of a non-specific time started in the past and still contines?

---

A set of four tiny combs from the Polynesian kingdom Tonga might be among the world’s oldest tattoo kits.

The tools had been sitting in storage in an Australian university for decades. A team of researchers recently reassessed the artifacts and found that the combs — two of which are made from human bone — are 2,700 years old.

Archaeologists have known that tattooing was practiced in several cultures since prehistory. Mummies from Siberia to Egypt have been found with tattoos visible on their flesh. Ötzi the Iceman, a 5,000-year-old mummy found in the Alps, has dozens of tattoos on his body, which some researchers think were inked on for therapeutic purposes.

---