IIRC OCaml compilation was super fast and the errors were useful and pointed to the correct line.

What is/was the problem with the current parser?

As the blog post says: "Error: Syntax Error".

Helpful error messages have not been my experience.

Syntax errors are suboptimal. They are usually reported at correct location and it's relatively easy to spot them.

On the other hand type error messages are simply awesome.

Generally auto generated parsers have huge problems with error recovery (bailing out quickly) and reporting. They usually exceed in performance however. I’m not sure where OCaml stands.

They do not want to change the parser but the parser generator.

One issue is mentioned in the blog post:

The days of Error: Syntax error. may be numbered

How is this a counter-example?

They move from a auto-generated compiler that sucks to another one (whose quality is yet to be seen).

Even if it proves to be great, it would still be an outlier.