What makes an internet an internet is that it's an interconnection of networks. In 1969, ARPANet is still only one network; by definition it's not an internet. So by that conception, maybe we should date the internet from 1977.
I skimmed that very article before asking my question. The internet isn't the kind of thing that really can be said to have one birthday, unless there's some kind of artificial-but-nonetheless-widespread consensus to declare one.
Wait wait wait, does InterNet have a precise date of birth?