Interestingly he uses pretty much the same techniques that I relied on when typing lecture notes with Word:
- Formula auto-buildup with automatic replacements of \stuff with Unicode keeps the mess of backslashes and braces more readable (and it also allowed me to immediately spot where I mistyped something)
- I used snippet-like things for parts I had to type a few times, such as a formula that's revisited a handful of times alter.
- Some things are simply easier to type in Word (with UnicodeMath) than LaTeX, such as a^12 being automatically what a^{12} is in LaTeX or fractions being written as a/b, thus requiring fewer braces. Also, as a user of a non-US keyboard layout back then, UnicodeMath's use of () instead of {} for invisible grouping and automatically resizing parentheses meant a lot less typing in general and less syntax to inadvertently break the layout.
The only thing I could not do in real-time was figures where I often did a quick sketch on paper and in the evening converted that into a nice drawing. But text and math was (at least for me) easily writable in real-time and generally required no post-processing or prettification. Assigning certain styles to keyboard shortcuts was also something the author probably used snippets for, but the end result for the typing experience is pretty much the same.
I did write notes by hand the preceding year, averaging about 7 pages of notes per lecture. Others have mentioned it too, the very act of writing notes helps remembering. For the handwritten notes I could just as well have bought the book by the professor, but I didn't do that on purpose. For me, there was no difference in being able to remember my notes between handwriting and typing. And generally I did spend most of the lecture writing and understanding came later when solving the assignments.