Written language is more subtle, more considered, more edited - he states himself that he writes then edits - in his case to make it more “spoken”. By doing this he is removing complexity in the interests of simplicity, and this may well fit with his goal for this work. It is not a general panacea.
I don’t disagree that sometimes it is more useful to have a simple introduction, leading to a more complex and better understanding of a subject before layering on the exceptions and subtleties - there is certainly a place for simplified knowledge transfer, our entire system of education is based on this “lies to children” approach.
What I do disagree with is that it’s a useful go-to rule. The world is inherently complex, and we deal with complexity by introducing layers of abstraction (more of the “lies to children” approach, but this time to ourselves). Not everyone needs to understand the quantum mechanical physics of a positive charge in order to understand that balloons will stick to your hair if rubbed against certain materials, but if you’re trying to explain that, then you read the room and go with the layer of abstraction needed. Sometimes that abstraction is very thin, and the language used will reflect that; at other times, “it just does” is the way to go… party handbooks printed on balloon packets are different to undergraduate textbooks.
So written language, with all its capability for complexity, context, subtlety and nuance should be employed when that capability has a useful effect. That means understanding one’s audience and tailoring to suit, not just a blindly-applied rule to “write as you speak”.