It's evidence of no such thing. It categorically does not take 12 years to write a language from scratch, or anything remotely close to that, so the only thing that Blow taking 12 years to write a language is evidence of is that Blow chose not to prioritise language development very highly over doing anything else with his time for 12 years. Javascript was famously developed by one person in 10 days. Writing a language and compiler is a common semester project for students, ie. people who have no idea what they're doing.

Famously, as with many other things in computer science, writing a toy programming language is doable in a couple of hours, but writing a usable programming language that needs to stand up to real programs for users is typically somewhere between a decade and a lifetime of work.

If he'd announced the language twelve years ago and then not touched it again until a few months ago I'd believe you. But there's plenty of documented evidence that he's been spending a bunch of time on it, and (allegedly) even using it in his commercial projects.

Not to say that he's been working on it full-time for 12 years, of course, but even allowing for part-time work that's far beyond the initial ‘easier than The Witness’ premise.