The 6,465 is curated advisories that apply to open source packages in the ecosystems listed. NVD’s 170,804 is all CVEs issued, many of which (the vast majority) don’t apply to open source packages.
(Not trying to claim the GitHub Advisory Database is perfectly complete - it’s not, and achieving that is part of why we’ve opened it up to community contributions. Just that the comparison with everything in the NVD isn’t apples to apples - the databases have different scopes.)