If it’s 1700 years old, that would date it to about the time that Helena (Constantine’s mother) went to the Holy Land to identify sites associated with the life of Jesus. No one knew where Joseph of Arimathea’s tomb was so they obligingly carved a fresh tomb to show to Helena as the right one.
Well maybe, and it doesn’t help that the author doesn’t seem very knowledgeable about the history of this church, and it’s not particularly well written.
Mean historic conservation practices were not quite up to modern standards. There was a temple to Venus on the site. They hacked away the entire hill, just leaving this core, if you will, where the tomb was located. They demolished the temple and built the church around the excavated core. A massive engineering feat. Now maybe Helena just took the word of some rando who said this was the site, but to undertake such a massive engineering project without some assurance it was the right site supposed quite a bit. Just as likely, the aging is correct and represents the romans just cleaning up and tidying the tomb to make way for the decorations to be installed