Nothing wrong with having modinstaller on your computer, but no mods can be installed on the game when you do runs. Including the modding API. The modding API makes changes to the vanilla game that's why it's specifically listed in the rules as not allowed.
Hollow Knight (patch 1.5.x.x onwards) is a game built with Unity 2020.2.2 and C# (.NET Framework 4.7.2), this means that a mod in the context of hollow knight is a dll or a Dynamic-Link Library that allows players to modify the behavior of the game, fix issues or add new features. This is achieved with the help of a Modding Api that handles the loading of mods and giving us ways of injecting our own code in the middle of normal logic.
The next few sections will give you a better idea of what modding hollow knight looks like beyond simple mods, Covering the concepts, tools & techniques that exist to make the job easier. It goes without saying that the Unity Scripting Reference and Microsoft .NET API browser are absolutely invaluable references when working within unity. In the context of hollow knight modding though, there are a few more concepts and resources that you want to be looking at.
Currently, Hollow Knight modders are editing the binary game file Assembly-CSharp.dll directly with dnSpy, and publishing the modified binary to Google Drive at a secret link that's shared in the #modding channel of the Hollow Knight discord server. This means that you can only be a Modding API developer/contributor if you are invited to the Hollow Knight discord server.
The part of the modified Assembly-CSharp.dll that is not copyrighted by Team Cherry is the modifications that the modding community makes. This is the Modding API. It is not illegal to publish the modifications, as long as the publication contains no content from the original file. 153554b96e