Explaining how to update your understanding feels like something that shouldn't need explaining, so I have no idea how well it'll turn out, but I'll give it a shot.
In the case of the original objection, the problem was instead of being willing to believe the other person commenting is being honest, and that they do actually have a manager so detached from reality, and what reasonable and fair behavior would be, that their manager expects them to respond to messages 24/7. Remember, we're assuming this person actually exists in real life. Either you believe this person exists, or you don't, or you actively refuse to believe that this person exists. For the above case, the argument was never you should pretend your boss behaves like that. The recommendation is that you assume the person you're talking to is not lying to you, until you have specific concrete evidence otherwise. If you don't have concrete evidence they are lying, you should believe them, and you should believe that [this person] thinks their boss expects them to be available 24/7.
This is an update to your world view. If you thought no boss could ever behave like this, your world view should now include there are plenty of people who believe their boss behaves like that. You don't have to discard your entire world view to say
shit, some people really do have a bully on a power trip instead of a manager who wants to help them be successful.
hell, you don't even have to believe that, you could just as easily update your worldview to include, wow, there are a lot more people who are afraid to say no than I thought there were. The point isn't to replace whatever idea you had, the point was always, listen to people and try to understand their point of reference, and try to use the things they have learned about life to help yourself, and if you're not incompetent, hopefully help them too. But seriously if you're still so stuck on how you just can not listen to them without completely discarding what you know today, pretend they aren't lying, but they're just engaging in some role play. In this magical make believe world, there is this evil bossman, how can we defeat him?!
If this idea, of picking up new information in the shape of an opinions from another person with eyes and the ability to form coherent sentences and complaints, is really so foreign to you... perhaps you do need to completely discard your current model for understanding, and replace it with a diff model for understanding that can be modified and updated in place without needing to start over.
You don't need to accept their reality as true, to believe them and want to help. Saying "that's not how my boss acts" is an attempt at a refutation, and a denial of their expression. You can doubt that their boss is really that bad, but still be willing to see if you are able to help, instead of rejecting them with, sorry I already believe something different and to even consider if you're describing something that actually happens, I'm going to assert my experience is exactly what it's like for everyone.