Yeah, looking at the linked study in the article, it seems folks who fought it off easily (often with low initial viral load) tended to be the ones who consistently tested blood serum antibody-negative. So if you had it and didn't get much more than a cough, there's a fair chance you didn't develop antibodies. Generic and local immune responses beat it in a lot of those cases (I gather), not virus-specific antibodies and a broad system-wide immune response.