PrismLauncher gives the following error when trying to install this modpack:

Exception: "'project_id's parent does not contain 'project_id'"
terminate called after throwing an instance of 'Json::JsonException'
  what():  'project_id's parent does not contain 'project_id'

Removing the top level projectID from the manifest JSON of this modpack fixes the issue.

After installing version 1.0.5 first, I am suddenly able to install the hotfix (without having to remove the projectID field). A bit weird, but I guess I can close this issue.

Strange, that file is built from packwiz and I've never had that issue before. I'll keep an eye out for any other oddities in the future.

It might also be a bug in Prism Launcher.

By the way, another unrelated problem I encountered when trying to use the hotfix is that I can't load any existing worlds because you appear to have accidentally removed Terralith.

Yes just download it again manually, will be fixed next patch.