That's not what they said though. They said "if you want to know if someone feels welcome, and you ask them, and they say they don't feel welcome, then you're done researching whether or not they feel welcome".
You are right. What it seems like a lot of people in these comments are saying is that the question perhaps should not stop at "Do you feel welcome?" because that only explores feelings of one side, which is highly subjective. If you invite someone into your house, they come in with muddy shoes and make a mess even though you had a sign at the door asking them to please wipe their feet, they might feel unwelcome when you ask them to go back, read the sign, and take off their shoes. The invitee might be offended, but the homeowner is justified and their feelings and the entire situation need to be considered. It is not "End of Discussion" once someone is offended.