The article is literally just not on point. They're confused about what NPS is for, and wrote a confused article about it.
I agree that it's very useful to track where your leads come from, and I love the short radio button lists that allow you to select a source.
If you feel like you should only ask one question, and it's either that or the NPS question, then it really depends on what your goals are at that point. Are you trying to figure out if your customers are happy with your product, or are you trying to figure out which of your marketing channels is most effective?
I think tracking NPS over time is really powerful, so then you're tracking not an absolute number, but you're tracking if customers have recently changed their opinion of you.