Loading Assembly Failed, Even Though It Shouldn't Be Loaded

As you can see here, I've got a dll that's set to only be used in the Editor, but when I start the game, I get an error that it can't load that assembly. Here's the full stack trace:

Code (CSharp):

1. Loading assembly failed: "Assets/System.IO.Compression.FileSystem.dll"
2. 0x00000001414141B8 (Unity) StackWalker::GetCurrentCallstack
3. 0x000000014141A666 (Unity) StackWalker::ShowCallstack
4. 0x00000001412FDE5B (Unity) GetStacktrace
5. 0x000000014014301E (Unity) DebugStringToFile
6. 0x0000000140BD9DB1 (Unity) MonoManager::LoadAssembly
7. 0x0000000140BD99F3 (Unity) MonoManager::LoadAssemblies
8. 0x0000000140BD3BCC (Unity) MonoManager::EndReloadAssembly
9. 0x0000000140BDD4E4 (Unity) MonoManager::ReloadAssembly
10. 0x0000000140E8088B (Unity) ReloadAllUsedAssemblies
11. 0x0000000140E7AA6F (Unity) HandleCompileStatus
12. 0x0000000140E76CCA (Unity) CompileScriptsWait
13. 0x0000000140E763E3 (Unity) CompileDirtyScriptsForEditorSyncInternal
14. 0x0000000140E76233 (Unity) CompileDirtyScriptsForEditorSync
15. 0x0000000140E768B5 (Unity) CompileScriptsForEditorSync
16. 0x0000000140E7E707 (Unity) LoadMonoAssembliesAndRecompileIfNecessary
17. 0x0000000141331741 (Unity) Application::InitializeProject
18. 0x0000000141498AD5 (Unity) WinMain
19. 0x00000001424831BA (Unity) __scrt_common_main_seh
20. 0x00007FFB90463DC4 (KERNEL32) BaseThreadInitThunk
21. 0x00007FFB92233691 (ntdll) RtlUserThreadStart

But since that dll is marked for Editor only, why am I getting this error when I hit the play button? Also, how do I fix this error? It seems to be harmless. I can clear the log and it goes away. I can play the game fine. It's just annoying that it keeps popping up.

Here's how I got myself into this situation: https://forum.unity.com/threads/c-compression-zip-missing.577492/#post-4419412