Not only that but microphone settings such as compression, noise gate, EQ would have an effect on sound and as you said, room size and other variances.
It also mentions “an important influence was how close the keys were to the edge of the keyboard” which who even knows if that would carry over to keyboards with plastic housings, etc.
Even in mass manufacturing keyboards will have variances causing a difference in sound.
This can also be completely circumvented by using a password manager which means people don’t have to type out their password at all.