The problem comes from the REFIID type parameter. It's not a GUID (a 16 bytes struct), but a GUID reference (REFIID), a pointer (so 4 or 8 bytes depending on process bitness).

So you could define the method like this, with a ref keyword (so the struct would be passed by reference, as a pointer):

internal static extern uint SHCreateItemFromParsingName(
    [MarshalAs(UnmanagedType.LPWStr)] string pszPath,
    IBindCtx pbc,
    ref Guid riid,
    out IShellItem ppv);

But I recommend this way which is easier to use and avoids creating/copying GUIDs all around (you will call it the same way as you do):

internal static extern uint SHCreateItemFromParsingName(
    [MarshalAs(UnmanagedType.LPWStr)] string pszPath,
    IBindCtx pbc,
    [MarshalAs(UnmanagedType.LPStruct)] Guid riid,
    out IShellItem ppv);