If anything, modern frameworks tend to build as much upon the native functionality that is available. This reduces bundle sizes and increases performance. It's an abstraction on top of the browser capabilities, not a clone.
Now, onto the code itself. The author claims it is "pretty simple" to build "fairly complex things". He backs up the claim by stating it took only 60 minutes to write this app.
First, I would be careful to claim things are easy or difficult. This is very subjective, and phrased in the wrong way it can also come across as arrogant.
Second, the app is not simple. Looking at the code the author created his own DOM abstraction. The current abstraction is only proven to work for a simple to do app.
Third, a to do app is not a "complex thing". Not even remotely.
It's a nice exercise, but the conclusion drawn by the author aren't valid.