I would imagine the distinction is that where, for example, you have a partner but you have care most of the time, you are likely to have someone around at the key moments (the birth! Birthdays. Christmas. Special days) and also you have the emotional support at other times.
I am a lone parent, and have been since my DS's birth. His father has never laid eyes on him. But I think this one sentence is grossly underestimating a lot of people's circumstances, married/in a partnership or not. You don't know their lives.
How often on here do you read posts about women who are being emotionally abused or gaslighted by their partners, who are terrified to voice an opinion? Who have partners who offer nothing or very little to family life. Maybe in a marriage where the partners feel equal, loved and listened to, you have a point.
Then again, I've also seen it time and time again on here that when a mother has finally gotten the courage to leave and her XH disappears never to have anything to do with the kids again, not paying any maintenance, they admit that while it's hard, they find it easier than the life they were living. So what does that say about it?
For what it's worth, I'm quite happy doing everything solo and making decisions for just me and DS. Yes, it's full on, and I never planned it this way - I was married when XH walked out, never to be seen again - but even the 24/7 adjustment hasn't worn me down.
I don't think it's a competition, and everybody copes differently.