In the passage, it states that Prometheus is ~5000 years old, whereas this new bristlecone pine is 5062 years old. It also states this bristlecone pine is the oldest known 'individual' tree in the world, thus meaning it is non-clonal. Although the passage only states that it is the oldest tree, not organism, we know that Prometheus used to be the oldest non-clonal organism. As the new bristlecone pine is older, we can assume it is the oldest individual/non-clonal organism as it is older than Prometheus. Hope this made sense.
EDIT: I think I know where you're coming from, but you might be overthinking it. Perhaps the passage expects you to assume that the bristlecone pine was the only old thing discovered that year? I'm not too sure.