From the 1989 article that you linked to:

"A visit by Steven P. Jobs, Apple's co-founder, to Xerox's Palo Alto Research Center in November 1979, is widely recounted in Silicon Valley chronicles as the inspiration for the Lisa and the Macintosh. It was then that he was shown Smalltalk, the first computer language using a mouse, the hand-held device used to instruct the computer. Several former Xerox employees worked on the Lisa and the Macintosh. [...]

Ronald S. Laurie, a copyright lawyer with Irell & Minella in Menlo Park, Calif., said Xerox's claim could be weakened because of the long delay in filing suit, some five years after the introduction of the Macintosh. 'There's a legal doctrine that you can't just sit around while someone's infringing your rights and not complain,' he said. In addition, he said, Xerox is 'going to have to show that when Star interface was published in 1981 there was a copyright notice published with it,' 'I don't know if it was published without notice,' he said, 'but I'll bet it was; in those days nobody put c's in a circle on computer screens.'"

Then there's PBS' 1996 "Triumph of the Nerds", in which the execs of the 80s are interviewed about the situation back then [1]:

"Larry Tesler: 'Everybody wanted to make a real difference, we really thought that we were changing the world and that at the end of this project or this set of projects personal computing would burst on the scene exactly the way we had envisioned it and take everybody by total surprise.'

But the brilliant researchers at PARC could never persuade the Xerox management that their vision was accurate. Head Office in New York ignored the revolutionary technologies they owned three thousand miles away. They just didn't get it.

John Warnock: 'None of the main body of the company was prepared to accept the answers. So there was a tremendous mismatch between the management and what the researchers were doing and these guys had never fantasized about what the future of the office was going to be and when it was presented to them they had no mechanisms for turning those ideas into real live products and that was really the frustrating part of it was you were talking to people who didn't understand the vision and yet the vision was getting created everyday within the Palo Alto Research Centre and there was no one to receive that vision.' [...]

Steve Jobs had co-founded Apple Computer in 1976. The first popular personal computer, the Apple 2, was a hit - and made Steve Jobs one of the biggest names of a brand-new industry. At the height of Apple's early success in December 1979, Jobs, then all of 24, had a privileged invitation to visit Xerox Parc.

Steve Jobs: 'And they showed me really three things. But I was so blinded by the first one I didn't even really see the other two. [...] I was so blinded by the first thing they showed me which was the graphical user interface. I thought it was the best thing I'd ever seen in my life. Now remember it was very flawed, what we saw was incomplete, they'd done a bunch of things wrong. But we didn't know that at the time but still though they had the germ of the idea was there and they'd done it very well and within you know ten minutes it was obvious to me that all computers would work like this some day.'

Adele Goldberg: 'He came back and I almost said asked, but the truth is, demanded that his entire programming team get a demo of the Smalltalk System and the then head of the science centre asked me to give the demo because Steve specifically asked for me to give the demo and I said no way. I had a big argument with these Xerox executives telling them that they were about to give away the kitchen sink and I said that I would only do it if I were ordered to do it cause then of course it would be their responsibility, and that's what they did.'

Larry Tesler: 'After an hour looking at demos they [Apple] understood our technology, and what it meant more than any Xerox executive understood after years of showing it to them.'"

And then there's an 1996 essay [2] by Bruce Horn, who worked
at both Xerox PARC and Apple:

"For more than a decade now, I've listened to the debate about where the Macintosh user interface came from. Most people assume it came directly from Xerox, after Steve Jobs went to visit Xerox PARC (Palo Alto Research Center). This "fact" is reported over and over, by people who don't know better (and also by people who should!). Unfortunately, it just isn't true - there are some similarities between the Apple interface and the various interfaces on Xerox systems, but the differences are substantial."

Lastly, a 2000 Motley Fool article [3] states:

"Xerox could have owned the PC revolution, but instead it sat on the technology for years. Then, in exchange for the opportunity to invest in a hot new pre-IPO start-up called "Apple," the Xerox PARC commandos were forced -- under protest -- to give Apple's engineers a tour and a demonstration of their work. The result was the Apple Macintosh, which Microsoft later copied to create Windows."

[1] http://www.pbs.org/nerds/part3.html

[2] http://www.folklore.org/StoryView.py?story=On_Xerox,_Apple_a...

[3] http://www.fool.com/news/foth/2000/foth000918.htm