Here is a paragraph from a school English textbook for high school students in Taiwan.
Apart from its natural splendor, Kyoto has so much more to feast one's eyes on. Having previously been the capital of Japan for over a thousand years, the city boasts a large number of historic shrines and temples.
According to the textbook, the second sentence is reduced from the adverbial clause:
Because the city had previously been the capital of Japan for over a thousand years, it boasts a large number....
I wonder whether the full form of the clause should be in the simple past:
Because the city was previously the capital of Japan for over a thousand years, it boasts a large number...
In the first and second paragraph, no information regarding other capitals of Japan before or after Kyoto is mentioned, so I wonder why the textbook says the second sentence is originally in the past perfect tense. I think the past simple should suffice.