It's worth bearing in mind that the source is Japanese. All input methods for the Japanese language are a compromise. Inputting kanji on a physical keyboard is nowhere near as fluid as inputting Latin characters - you're constantly toggling between inputting kana and selecting the kanji options presented by the IME.
The prediction and correction technologies of smartphone keyboards are a very good match for kanji and hànzì input. As a second-language user of Chinese, I find it considerably faster and easier to input hànzì on a smartphone than with a physical keyboard. The context switching between inputting pīnyīn and selecting hànzì is much less expensive when the hànzì are presented directly above the on-screen keyboard. The prediction and correction algorithms seem to be far more intelligent on mobile, which largely compensates for the slower and more error-prone tactile experience.
It is my understanding that most young Japanese people prefer the flick input method, which is a refinement of the old keitai input method used on featurephones with numeric keypads; they are often startlingly quick at using this method, but it poses a far higher switching cost when moving to a QWERTY-derived physical keyboard. I find it entirely plausible that the flick method could simply be inherently superior.
https://www.youtube.com/watch?v=8V-za9LT_30