The URL you gave has the R21.5 for download. You could use JetPack, or directly use the driver package plus sample rootfs. JetPack is a front end for the driver package plus sample rootfs, and has other features in addition (such as making available other software to both host and Jetson). You do have to be logged in for some of the software to download.
Essentially the driver package is for flashing, the sample rootfs is the actual software to be flashed. The driver package does do some adjustments to the generic sample rootfs before flashing.