Like how Wing Commander II is the addon to Wing Commander I!
Secret Ops is not an addon, it's a stand-alone release.
Secret Ops is most definitely a stand-alone release, and is most definitely not an "expansion pack". But the thread poll didn't ask about "expansion packs", it asked about "addon"s, and I would argue that the definition of "addon" is subjective. While I don't doubt that the pollster likely meant "expansion pack" and is therefore in error, I think calling it a "stupidly wrong" mistake is taking a rather narrow view.
As to the WC1-WC2 thing... I thought about that, but I don't think any but the most ridiculously wide view would consider WC2 and addon to WC1. Look at my criteria. WC2 uses a similar, but not the same engine, as WC1. I imagine a significant percentage of the source code was re-written. Some of the game mechanics (phase shields, capship attacks, torpedoes) are dramatically different, leading to a very different gameplay experience. The artistic look of the game was dramatically changed--compare the ship designs or the animated character avatars. And, story wise, WC2 starts ten years after WC1, after most of the characters from WC1 have been killed or have moved on, which separates the story and allows new ships, new weapons, new technologies, and new characters to be present. Finally, WC2 has a great deal of background and a lengthy intro that makes it possible to jump into the game never having played WC1.
On the other hand, SecretOps used essentially the same engine as WCP. There are no stunning new capabilities (heck, even Secret Missions and Special Ops introduced more new material, gameplay-wise, than Secret Ops did, because they added new flyable ships and weapons like the Mace with dramatically new capabilities, instead of re-using existing things with slightly tweaked stats). It looks like a large part of the game source code--everything except the bits relating to the story--were re-used wholesale, with only possible minor tweaks to some of the statistics. All the same graphics and models were used. The artistic look of the game was essentially the same, and the differences were mainly omissions (no cutscenes) instead of additions. And the story is set immediately after WCP, with mostly the same characters. Finally, to really understand what was happening in SecretOps, it's kind of necessary to play Prophecy.
I think, if one uses a loose definition of "addon", WC4 is far more likely to be considered an addon to WC3 than WC2 is to WC1. But Secret Ops is the most likely to be considered an "addon" of any of the "standalone" games.