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> or it was because I logged into Google+ using the default Android Google+ app and it enabled it then

It asks you if you want the uploads to take place when you first setup the app.

Unless its a small, easy-to-miss checkbox, I was only asked to log into my Google Account.

it's definitely not small or easy to miss. The whole "instant upload" part is an entire screen outlining what it is with a clear opt-out.