The linked answer is obsolete since the Rock Generator is now part of Extra Objects add-on since commit rBA7347930a0fa14e752929e28c9b48adf74597b018. The separate Rock Generator add-on has been removed from the add-ons contrib repository by commit rBAC70b649775eeeebedb02c1c7b7aa996a7f6294177.
If you are using a 2.82 alpha build from the 12th of December or later, enable the Extra Objects add-on. The Rock Generator operator will appear in the Add > Mesh menu.
If you are using a build of 2.82 which doesn't include the commit, because it was compiled before the 12th of December, you will need to download a more recent version.
If you don't want to use the latest 2.82 alpha which includes this change, you will have to clone the repository and either manually replace your older Extra Objects add-on with the updated version or revert to a commit where the Rock Generator was still a separate add-on. Both options require experience with git.