The people who made that game worked on Portal. Valve hired them after they saw the game, picked up the concept, then fleshed it out into a full retail game. I wouldn't say that project explores the concept enough to be an alternative to Portal, though I recognize it's importance to the development of something much greater. And even if Portal is or isn't original, it's still a really solid game. As you said, originality doesn't matter as long as the game's fun, and I happen to think Portal was very fun. I'd give it a B+.
And since this still isn't the argue about games thread, I'm gonna talk about another game that I'm playing right now.
Castlevania: Order of Ecclasia is pretty rad. I just made it to Dracula's Castle, which is pretty intense, since it's probably the same size as every other level combined. The glyph system is really awesome, but I wish it gave you more to collect to fight with. I love being able to finally combo monsters, and the steep difficulty doesn't bother me nearly as much as I thought it would. It's hard as balls, but managable.
Great game, but I'm getting a little tired of these "Metroidvania" games. I know they're the only ones that sell anymore (lolJudgement) but they haven't done anything radical to the main series since Symphony of the Night.