The budget overruns are predictable. To the point where you can confidently predict that neither year nor the budget will stay as they are currently predicting. Honestly, it sounds more like wishful thinking at this point. I would say late 2030s at probably > 40 billion is realistic. They'll find a way to make it worse. Maybe they'll get lucky and only overrun only a few billion. Every year it overruns will cost billions.
The reasons listed in the article dance around the real underlying reasons and causes which relate to nuclear plants being a once in a generation kind of thing at this point. Most of the people executing these projects are on their first nuclear project. And they have to re-learn a lot of the things that make these projects complicated. There is no learning effect between projects. And in so far there is, it seems to be a negative one. And by the time they are done, there's a new generation that needs to build the next one.
The timescale doesn't help either because lots of things change and assumptions get broken. For example the relationship with China looks a lot less cozy than it did ten years ago. And that might change substantially again in the next ten. So them being a major partner in this project complicates things. In the same way, the relationship with Russia changed so relying on them for supplying the fuel rods might not be as good as an idea as it was back when Finland started planning its plant (the predecessor to this one). It's hard to predict these things on a multi decade scale. So, we're talking major changes with suppliers, project participants, and probably technology as well. And the competition.
The reason renewables are running circles around everything else is because planning cycles are short (Hinkley point C is about 3.4 GW of energy. Not nothing. But the UK has added a multitude in wind and solar since planning started and will add another multitude of energy by its completion (whenever that is). It's not even going to be close. Coal had a major market share when planning started (more than nuclear). The last coal plant in the UK closed last month. By the time this nuclear plant opens that will be ancient history and most of the grid in the UK will be wind, solar, batteries, imported power from abroad (cables), and a few gas plants. The good news would be getting rid of those remaining gas plants.