I've always taken it to mean "coding is the easiest part of a developer's job"

That doesn't make it inherently easy, but it is the easiest part to automate.

A modern LLM is perfectly capable of maintaining decent code quality and architecture (provided you ask for it) up to a few thousand lines of code, but after that it very quickly loses the plot if you're not designing your documentation right and keeping a hand on the architectural tiller.

Architecture is about staving off chaos for as long as possible given the maximum functionality you expect it to achieve. That's hard enough for a human, with a deep understanding of your business, to do. Architectures that endure is a hard problem, dwarfing the difficulty of writing the actual code. Choosing what product to build is also a hard problem if you expect to meet any success. What it does, what it specifically doesn't do. Sounds easy on paper, and if all you're doing is Sunday prototypes it feels almost trivial. But once you're doing a real product with real consumers, it's a very different thing.