The reasoning is that when you produce output you are by definition producing something at your own level, so little improvement occurs. When you consume input (and transcription is a great way to do it), you are consuming input of a native speaker - or in the case of Jazz, a master player - which is at a much higher quality level than what you could produce yourself. So input drives the learning process.
The exception is for motor e.g. pronunciation or playing technique, which do benefit from practice.
Been prepared that it will sound absolutely terrible for the first few months or even years.
Probably also more common among jazz musicians than rock musicians, who don't emphasize sheet music and music theory as much (as much, it's certainly more prevalent than outsiders would naively think).
And of course, all these categories blur together. There wasn't any reason to take his post and interpret it in absolutist terms.
One of the authors she mentioned was Yasunari Kawabata, who is widely admired for his writing style in Japanese. I believe she also said that she got the idea to do this from the tradition of shakyō (写経), the copying of Buddhist sutras by hand [0].
I have never retyped literary texts myself, but I used to teach English literature classes to Japanese students in which I would read the texts aloud in class before we discussed them. I found that, over the years, that experience heightened my awareness of writing style and maybe improved my own writing as well. I can see how retyping might have a similar benefit.
Do you mean learn other people's songs from sheet music, or do you mean write down, from the sheet music, the same song again on blank sheet paper?
Edit: Or do you mean write down the song via hearing?
You can buy books of solo transcriptions, but I've found that they're absolutely useless. (and always inaccurate) Maybe it's like reading a book vs. typing it out?! hehe. At first glance I couldn't see how typing a book out would help anything, I mean, it's already written down for you! The main element of learning by working out bit by bit what it is, isn't there. But there are elements in common - it's a little like playing a solo you've written down, and you would absorb the word patterns if you typed a whole novel I guess.
Maybe he means like jazz musicians? (I'm one.) Every musician I know has transcribed solos. You pick some solo you like but have no idea what they're doing, transcribe it, and learn to play it along with them. Bits and pieces perhaps will rub off in your own playing. You don't want to sound like them, but it's good to be able to if you want – and for that you need to absorb their style, so your body can just go into that mode, without having to think about it. I'm a piano player but have also transcribed sax, trumpet, bass lines, gospel songs, reggae, funk, .. even taps dripping, babies crying etc etc.