One way* to come up with an estimate of how many leaves - or needles, in the case of Sequoiadendron giganteum - is simply to count the number of leaves on a twig (or a number, to get a good average), then the number of twigs on a branch, and then count the branches on the tree, after which it's just multiplication.
Now one reason that the number seems so high is the way the needles grow. Unlike for instance pines, which have long needles arranged in sparse clusters of 2, 3, or 5, or spruce & fir, which have medium needles arranged along the branches, the sequoia has lots of tiny needles arranged on twiglets.
Link with picture of sequoia needles: https://www.monumentaltrees.com/en/trees/giantsequoia/giantsequoia/
Picture of pine vs spruce & fir: https://www.finegardening.com/article/fir-vs-spruce-vs-pine-how-to-tell-them-apart
*But I don't know whether it's the way used to get the number in the link.