Right below the line you quote, the author makes three arguments:
> • It’s highly instructive, in a way that using a parser generator is not. To quote Feynman: “What I cannot create, I do not understand”
> • It’s an important skill: most real-world compilers use hand-written parsers because they provide more control over error handling, significant whitespace, etc.
> • It’s not actually difficult!
The exercises themselves are meant to justify these arguments.
I think the exercises conclusively demonstrate that #1 and #3 are true. If you can easily follow these exercises, and if, by the end, you feel like you've learned something useful, then it follows that hand-rolling a parser is instructive and not too difficult.
I strongly agree with the author that if you're developing a language for the first time, you shouldn't use a parser generator at first. You should hand-roll a parser first, and then, when you see the limitations of your hand-rolled parser, adopt a parser generator, now with full understanding of what the generator is doing for you (and not doing for you).
As for #2, the article demonstrates how to do error handling in a manner that would be a head-scratcher in Yacc.
You claim here that it was easier to develop a "human friendly" syntax with megaparsec than it was to do that in a hand-rolled recursive-descent parser. That could be true, but that's not my experience. To be convincing, you'd need to do what the author did, (even though you say the author didn't): you'd need to justify your argument with specific examples.