if this was true, then leetcode wouldn't be the cost of admission?

Writing code may not be the ultimate objective of software developers, but it is one of the primary tools to achieve that objective.

The article’s argument was that the company was able to migrate away from his code quickly, so his code didn’t matter.

That’s…an argument. I think most developers, myself included, find the idea of migration to be almost impossible in many cases. The author handwaved that away too easily.