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.