Amazing question! Yes, but also no.
2 of the grip of people that said it to me, said it like it was a curse. Like the only lesson they leaned in their whole damned lives was to treat their partner like a partner. Ruefully. Regretfully, they learned their ego wasn't enough, and admitted it in a moment when they needed to foist their greatest insight on a relative (or relative's kid's fiance), and that's just what spilled out of their arrogant Boomer mouths.
3 definitely said it like "this is actually the rule to know." An insight they shared neutrally. Well, one more like a 1000 yard stare.
One, and one alone, asked if I knew it. My spouse and I got married after 5+years together, so it was a fair question. I knew already.
It's more like a swear, to either the gods above or below. Take your pick.