History does not start with XP 

Continuous integration has a longer history than this article indicates.  Integration has been performed on software projects for a long time.  A number of the elements that are now seen as continuous integration were described by Booch. 

In the context of continuously integrating a system's architecture, establish a projects rhythm by driving to closure certain artifacts at regular intervals

He then goes on to say there are three parts to this:

First, the macro process of object-oriented development is one of "continuous integration.". Rather than set aside a period of formal testing at the end of the life-cycle, the object-orientated life cycle tends to integrate the parts of software (and possibly hardware) at more regular intervals...

Second, at regular intervals, the process of continuous integration yields executable releases that grow in functionality each release...

His final part is that by having integration more frequently quality can be assessed by management, risks can be identified and acted-upon early.