It matters if you are serious about improving your UX processes, even if only from a philosophical point of view that you can't improve something if there isn't a standard or benchmark to measure your progress against.
I think in the earlier days of the UX field (even though many see it as a continuation of HCI) when organisations and UX practitioners had to deal with the reality of fitting UX processes and methodologies into the traditional methods of software development, the last thing they would have been worried about was whether they conformed with the various ISO standards that applied to the products and services that they were working on.
Also, it is very common for standards to not be aligned to what is considered 'best practices' in a rapidly changing IT industry, so I would not have been surprised that the answer to this question when it was posted 7 years might be different to the answers that we get today.
Considering that WCAG 2.0 also forms part of ISO 9421, and that many parts of this standard have also been updated and adapted so that is is more in line with modern trends and practices, I think it is now a very good document for either benchmarking the maturity of an organisation's UX maturity or for implementing a design/development framework with a human-centred design approach.
One of the down side of the emphasis on lean and agile software development practices these days is the lack of rigour and structure involved in many UX processes. This has led to issues such as research debt, technical debt and a multitude of UX tools claiming to solve the kind of problems that you can only really solve by having a sound design/development framework. I believe that ISO 9421 is comprehensive in the depth and breadth of its coverage on topics relevant to UX design, and can be extended or customized for the specific requirements of each organisation.
Of course, many existing design or development frameworks (and playbooks) overlap with topics and concepts covered in ISO 9421, so as with most question in UX, it is certainly best to assess the problem that you want to address and work out what the best solution given the constraints. Even so, I believe that ISO 9421 is definitely more relevant today than it might have been in the past, so it does matter.