It could easily be 100% Islamic terror. Unless he wrote a manifesto or something, we'll never know if he had a personal animus for his job or co-workers beyond their nominal infidel status.
Yes, it could be a two-fer, however, it could also well be just a place that he was intimate with the layout, the (lack of) security, and could guarantee himself and the other two shooters where a large gathering of people would be.
The other Middle Eastern housemates, whom I'm unclear how many there were, and if their whereabouts are known, and the pipe-bomb IED workshop, RC cars/toys with bombs on them etc. definitely points to it being majority Islamist terror, and minority workplace revenge/violence at best, if at all.