Earlier quoted context omitted.
What does agile have to do with anything? You think waterfall explicitly provided time to test out the UI and "sand" it? This is a process that generally requires a product manager to choose to prioritize, together with a capable UX engineer and/or designer. That prioritization can be inserted into any development methodology if you want it. Agile is irrelevant here.
Agile is relevant here because it has been adopted almost universally as a shovel to push more things on the programmer's back without much care to the quality of the changes being done. Sure, it is not to be blamed as per Agile manifesto but how many companies are adhering to the manifesto? It is how it is used, not what it was used for.
Increasingly, I am also blaming the Agile Manifesto for causing the mess we are all in.
Given current business tendencies, the authors should have probably foreseen the future consequences of their wishy-washy manifesto declarations. I think "true Agile has never been implemented" should not absolve them from assuming some responsibility.