It's a misunderstanding on your part, caused by reading documentation intended for Debian and not Ubuntu
The "allow-hotplug wlan0" line, on Ubuntu, should read "auto wlan0"
** Changed in: udev (Ubuntu)
Status: Unconfirmed => Rejected
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ifup invoked as --allow auto instead of --allow hotplug
https://launchpad.net/bugs/53829
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