Although I agree that chess is 90% tactics when actually playing at the board, it really depends on where you are strenght-wise as to how much time you spend on tactics. By the way, when they use the word "tactics" there, I would say "tactics and calculation".
When you are beginning, I firmly believe that spending 90% of your time on tactics will be the best use of time, and you will get your biggest bang-for-the-buck time-wise. I once guided a 38-year-old from 1000 to 1850 on ICC in just three months by having him do 50 tactics problems per day, spending no more than two minutes per problem. He did nothing but tactics during that time, and play. I firmly believe in that immersion method of learning.
As you get stronger, let's say, 1800, you need to start spending more time learning about position. That will continue forever, but it becomes much more refined as you get stronger. You should still spend a lot of time doing tactics. I have found that even 2100 players are not that strong tactically, and even as a 2200+ for decades, I miss too much.
As you become a strong master, in particular GM, they need to work on openings more than any other group since so much is won at that level based on preparation.