But there's a huge difference between writing a compiler for a real programming language and doing a few regex or xpath queries on some data and then doing something with it. It doesn't have to be an expensive engineering project, either. The difference is in the scope, and it's an important difference. Building a compiler is a comprehensive task-- it requires a solid understanding of the regex language of your tokenizer, it requires a solid understanding of syntax grammars and recursion, it requires you be able to traverse a large, complex syntax tree to generate code. You will almost certainly explore methods of helpful error reporting that rarely come up in typical programming (example: error productions). No, it's not beyond the reach of any decent programmer. But it's going to be a lot of work. The essential complexity of compiling is significant.
The difference between implementing a classic compiler for a programming language and throwing together a calculator parser is like the difference between writing a 30-minute symphony for a full orchestra and a beginner piano solo. Sure, the fundamentals are basically the same, but the experience is totally different.
And that, I believe, is the point of the essay. If you write a symphony, writing each line for each instrument as if it were your own, keeping track of all the dynamic contrasts and interplay between the parts while developing a coherent structure to the piece as a whole as it develops over a half an hour, writing a simple 3-minute verse/verse/refrain pop tune for guitar and vocals will feel like nothing. If you build a compiler, you'll be able set up a simple regex parser for a configuration file with blindfolded with one hand tied behind your back.