I would agree that sentence comes off as somewhat misleading, but it doesn’t actually say you can use Seek to add observations.
Users can access iNaturalist data or add their observations to iNaturalist in several ways: via the iNaturalist.org website, through two apps: iNaturalist (for iOS/Android) and Seek (iOS; Android app is in beta testing as of January 2019), or through partner organizations such as the Global Biodiversity Information Facility website.
It’s saying you can use those four platforms (iNat website, iNat app, Seek app, GBIF website) to either add observations or access iNat data. The next sentence actually clarifies that Seek data doesn’t go into iNat.
On the primary iNaturalist app, users can contribute nature observations to the public, online dataset, though on Seek, which was designed for children and families, no online account registration is required, and all observations remain private and are not uploaded to the dataset.