I personally have not found the UX of HBO's streaming offerings (I use HBO GO, primarily through an iPad and a PS4) to be appreciably worse than Netflix's. The apps load and are straightforward to find my way around in, I can always find the content I'm looking for, I've never had any technical issues prevent me from streaming that content.
In some regards, I would even say their UX is better than Netflix's -- scrolling through a list of items in the HBO apps doesn't trigger a parade of loud auto-playing videos, and HBO doesn't visually privilege their own original content over stuff they've licensed by giving it huge icons and putting it at the top of every list.
The article mentions that HBO couldn't handle the streaming demand for Game of Thrones in 2014, which is pretty bad, but which was also five years ago. It doesn't mention them having problems like that anymore, so it seems reasonable to assume they learned something from the experience.
If anything, I would say HBO's biggest failure is in marketing/branding: naming their offering for cable subscribers "HBO GO" and the one for cord-cutters "HBO NOW" is an invitation for confusion, and the logos/icons for the apps are so similar anyone could be forgiven for downloading the wrong one. They need to distinguish these services better, and provide some incentive to use HBO Now to attract people like me who are getting HBO through their cable company to jump to having a direct relationship with them instead.