That has not been my experience in decades around pipelines.
If it’s seamless pipe it’s probably in the weld and someone missed it with the X-ray.
An initial construction defect that did not develop a problem until after after 20 years?
With 20,000 barrels there had to be fluid cutting over time.
How much cutting and over how much time we don’t know.
It took time to spread over that large an area.
With a 1/4’ hole all at once, it’s hard to believe oil wouldn’t have been on the surface much quicker.
It may not have been an actual “defect” as it was an “imperfection”, two completely different things when your talking about pipe inspection.
Weld “imperfection” isn’t any different than any other “imperfection”.
It isn’t a problem initially but over time it can develop into a problem.
For example a small pit on the ID isn’t a problem, it’s an imperfection that will meet the specs, but over time swirling fluid inside the pipe can cause it to expand and deepen until it is a problem 20 years later.
Pipe inspection isn’t 100%, even when pressure testing, you can still miss it.