It's fairly simple: in the script, just replace the lines

	ignoring application responses
		tell application "loginwindow" to «event aevtshut»
	end ignoring

with this single line

	tell application "Finder" to eject disk "YourTimeMachineVolume"

(obviously replace YourTimeMachineVolume with the real name of your Time Machine disk)
Moreover, if you want the script not to launch the backup (because you prefer to launch it by TM menu) but only unmount the volume when Time Machine has finished, remove the following line from the script:

   do shell script "/System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd-helper > /dev/null 2>&1  &"

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