Something wrong with the assumptions in the article
Samsung executives know that the competitive threat comes not from Apple, but from an ever-improving, ever-evolving Huawei.
Like you I think that the market for the most expensive phones is limited but I also think that the biggest threat for all manufacturers comes from the "I'm happy enough with the one I've got brigade" who won't buy a new phone every two years. Pretty much all the highend phones of the last few years are technological marvels but even so they have hardly moved the needle on the day-to-day experience. With the S6 Edge Samsung did discover that there was money to be made in aesthetic improvements and volumes for the subsquent highend phones would suggest that there is a fairly large market that will pay a premium for the styling (and the underlying technology). It has also got better at selling cut price versions of the highend devices so that it wins whichever way the market goes. Apple is now very much playing catchup technologically even as it maintains an edge with the GPU and CPU.
It's the same with DeX. If Samsung can do this well and fast enough it stands to do well in sales – might be only a couple of % of the total but still millions in absolute terms and will reinforce the brand because this is something that people will pay for.
And I do think the brand is worth more than you suggest. From my own experience, but also anecdotally from the posts here, the reason for switching seems to be as much about customer support (and software updates) as it does about price or technology. Personally, I'll always be looking for a phone with good AOSP support.