"Signing up" implies creating a few database records and logging someone in. When I say "onboarding", I mean a deliberate attempt to transition somebody into the workflows and uses of the product.

I believe 'on boarding' refers to post signup new user education. That appears to be how he used it here. I have never heard it used as just signing up.

What do you call post signup new user education?

I think that, in an article directly related to marketing, marketing-speak is appropriate.

Signing up new users != onboarding. Getting them to signup is only half the battle. Onboarding, or getting them to actually setup their account, start using the product and usually taking actions that offer the most value is a pretty important part of reducing churn.