Im not at all surprised that they learned at the same rate. Their English reading was bootstrapped by the German.

Likewise, I could mostly read English when I arrived in the new country despite not being able to say or understand a single word. This is because English is, maybe, 75% phonetic and I had already figured reading out.

Reading is not just memorizing characters and phonemes, but being able to join everything into a fluid mental motion. In english, practice of their fluidity is constantly being interrupted by an adult having to tell the kid which sound variation a vowel has