While it is true that English is mutable across time and non-standardized across cultures, for an individual learning English as their first language, they are learning a snapshot of it that is--from their perspective--immutable and standardized.

The article's argument is the lack of consistency, not standardization. Unless you are arguing that children--in whom dyslexia first manifests--are being exposed to the vast diversity of English across time and space, then there can be no doubt that mutability and lack of unified culture contributes nothing to dyslexia.