The KB4516421 update is for the 1903 release only but also only offered for certain machines with USB audio devices attached.

If running the 1903 version but with no usb audio devices connected to any usb port, then the KB4516421 update will not be offered thru WU.

btw, KB4516421 is available only thru windows update and windows server update services (WSUS) and will not be issued thru the MS update catalog (MUC) site.