I don't think they're implying that. In fact they said "It can be impossible if both of you work" which you can extrapolate to "It can be impossible if you're the sole caregiver".
Of course if you are the only person home, you can't expect a 4 year old to let you work uninterrupted for long stretches of time. I don't see anyone trying to argue that point of view though (certainly I would never say that).
It’s that as a parent it’s hard not to just have some part of your brain constantly scanning for any sign of your 4 year old trying to get themselves hurt or break something (as 4 year olds naturally do).
In other words, the boundary crossing is done by the adult and is really hard to turn off if you have any interest in your kid staying safe, fed, and entertained throughout the day.
Establishing boundaries like "don't interrupt the person who's working" needs to be a discussion that happens between the whole family. Of course, the caregiver will be responsible for the physical enforcement but the caregiver should understand that it is in the long-term best interest of the whole family that the worker is able to have uninterrupted work time.
Also, by the age of 4 most kids are capable of understanding and learning to respect boundaries.