If you report being satisfied with your relationship to a third party because you think all relationships are like your current relationship and you don't see its dysfunctions, are you really more happy, or just more ignorant?

Happiness is a feeling. If you feel happy, you are happy. Ignorance is irrelevant, it may even be beneficial ("ignorance is bliss").

It's definitely not that black and white, because they're not directly measuring people's happiness, they're asking them to self-report their relationship satisfaction.

If I had a mediocre partner but I thought that was just how relationships were, I'd probably report being "satisfied" with them, even as I wished relationships weren't such frustrating things. If I thought marriage was an unbreakable bond I'd probably also avoid admitting I was dissatisfied with my partner to anyone (even myself!) as a coping mechanism as well.

The opposite could also be true, that people avoid admitting regret over their promiscuous past by convincing themselves that the "experience" makes them better judges of the quality of their relationships

There is no possible measure of happiness beyond self reporting, unless you are a dictator who has decided that all your subjects are extremely happy.