I have found the issue.
You will have to wait for @texnixe to push an official release but for now you can fix it by fixing the typo in the plugins
line 6 should read:
'texnixe\\Spreadsheet\\SpreadsheetLoader' => 'classes/SpreadsheetLoader.php',
The filename isnt tottally CamalCased so either fix the filename, or fix the line that loads it.
On a side note, I couldnt get the plugin to install via composer, so i did it manually and then went into the plugins folder in terminal and ran composer… perhaps that helped too.