Good question!
Writing or calculators likely do reduce our ability memorize vast amounts of text or do arithmetic in our heads; but to write or do math with writing and calculation, we still must fully load those intermediate facts into our brain and fully understand what was previously written down or calculated to wield and wrangle it into a new piece of work.
In contrast, LLMs (unless used with great care as only one research input) can produce a fully written answer without ever really requiring the 'author' to fully load the details of the work into their brain. LLMs basically reduc ethe task to editing not writing. As editing is not the same as writing, so it is no surprise this study shows an serious inability to remember quotes from the "written" piece.
Perhaps it is similar to learning a new language wherein we tend to be much sooner able to read the new language at a higher complexity than write or speak it?