I disagree with this article largely because mechanics and gameplay can do a lot to save a game sequel even if the sequel doesn't add much to the story. There's series like Mario where the stories are total repetitive nonsense, but the games continue to be good thanks to Nintendo constantly adding and refining mechanics. Pokemon is another good example, where the story changes minimally between sequels, but virtually everything else is improved. Sometimes even a graphical update is enough to justify a sequel. There are franchises to which story is very vital and needs to be a strong consideration when making a sequel, but there's also plenty where I'm find with a bit of nonsense in the story department for the sake of improvements the rest of the game's design. There's more than one valid reason to make a sequel, and some franchises just aren't as story dependent as others.
That said, story is very important to Portal, but I have yet to feel like Portal 2's story or GLaDOS's inclusion in it have been forced. The game is solid when it comes to mechanics and pacing. The new "toys" in Portal 2, such as the various goos, excursion tunnels, and hard light bridges, expand well on the mechanics of the first game and add loads of fun. I haven't gotten bored with testing at all. The game is plenty broken up by the different sections of the facility, "off rail" segments, and by the additions of Cave Johnson, Wheatley, and the changes in GlaDOS. The atmosphere changes a lot as you move forward. All in all, I felt Portal 2 succeeded as a sequel by subtly expanding on the history of Aperture Science through more excellent character dialogue, with lots of humor and mind bending puzzles to keep the player engaged.
Portal 2 has amazed and entertained me immensely. I know fan pressure can negatively influence franchises, but I see no evidence of this in Portal 2.