I used to do a lot of white water kayaking and when I read about the story I immediately knew what kind of weir the one in Haverfordwest would be. First photo I saw confirmed it.
That kind of weir, with a vertical drop off, is notoriously dangerous. So much so that they are colloquially known as 'drowning machines' among white water paddlers. Even without the river being in flood, it's an extreme hazard because the water circulates and drags you constantly back towards the weir. With the river in flood it's only luck that they didn't all drown.
It's not a matter of experience, these weirs are extremely dangerous no matter how much experience you have, as demonstrated by the death of the instructor who was very fit and experienced in watersports.
You always come out and 'portage' (walk with your watercraft) round a hazard like that. The fact she didn't do this suggests she had absolutely no idea of the risks of moving water. Honestly, it is pretty much the first thing you learn when you do the most basic moving water safety course.