No, they don't have as much mental load, because they are only having to do that for themselves. They have more mental load than somebody who is healthy with no dependants, but they don't have as much mental load as someone in the same circumstances who also has to manage dependants.
I don't see why you aren't getting this. A healthy adult with no children has a lower mental load than their equivalent. You almost seem to being seeing this as some sort of slight. I don't know why.
It's a damn sight easier for me being a grown up without children, even though I still have to do all those things for myself, the point is I am not doing them for someone else as well.. It's that simple.
At the moment, I am staring down the barrel of increased care for my elderly parents that will increase my mental load, but it still won't be as high as someone in that same situation who also has toddlers. I am not going to have the extra mental load of ensuring my children are fed, watered, at school/nursery on time, etc, whilst at the same time fitting in getting my mother to appointments, etc.
I've only got to worry about my mother and myself. That's easier than having to sort out my mother, plus two young children, etc. And if, then, on top of that, my husband was asking me when his next dental appointment was, I might just explode.
I can be a grown up for just myself with little trouble. The more people I have to be a grown up for in addition the more load I have..