It also showed that people couldn’t successfully recall information about what they’d just written when they used LLM assistance.

Writing is an important form of learning and this clearly shows LLM assisted writing doesn’t provide that benefit.

No one is surprised by this, though. Naturally, when you write, you will learn to write. When you make an LLM write, you're going to learn how to make an LLM write (well).

The question is how well your assumption holds true that learning to write generalizes to "an important form of learning".