Character amnesia is a unique phenomenon to hanzi/kanji and is absolutely incomparable to English and French orthography. Native Chinese and Japanese speakers learn characters largely through muscle memory, which deteriorates rapidly if you don’t regularly write by hand. Many university-educated adults are unable to write a supermaket shopping list without hesitation.[1] Conversely, in English I learn difficult spellings (Gloucestershire, syzygy, rhythm) by aural-mnemonic tricks which stick around in my memory essentially forever.

Whether this had any effect in this study, I don’t know, but it certainly seems relevant that the experience of writing by hand in Japanese is utterly different to any alphabetic script.

[1] Don’t take my word for it, here’s an interesting blog post on the topic: https://languagelog.ldc.upenn.edu/nll/?p=2473