Can you access files and subfolders in an inaccessible folder on Windows?

Question

I want to make a folder and all its content readable and writeable to administrators and the local system account only. Currently, I do this by the following C# code:

var ds = new DirectorySecurity();
ds.SetSecurityDescriptorSddlForm("O:BAD:P(A;OICI;FA;;;SY)(A;OICI;FA;;;BA)​");
var di = new DirectoryInfo("Myfolder);
di.SetAccessControl(ds);

After this, could there still be some file in "MyFolder" or its subfolders that a normal user could read, write or delete?

I assume the normal user is a hacker that could originally have created the "MyFolder" folder and placed content in it in any way that normal user privileges allow.