To set your expectations correctly, MATLAB Support package attempts to install its own Arduino IDE which is 1.8.1 and that is the one it is supposed to find when you type arduinoio.IDERoot.
If the output is just arduino1.8.2, there could be an installation issue. Can you try uninstalling the support package and reinstalling them with administrator access. You can do that by right clicking on MATLAB icon and selecting Run as administrator.
After this you can check again with the same command and the default location is inside C:\ProgramData\MATLAB\Supportpackages\<Release>\3P.instrset\arduinoide.instrset\arduino-1.8.1