I am in no way suggesting that ink is king, and is the only medium one should ever use. To put it simply, ink - and specifically fineliners - are the tool that pair best with these lessons and the principles upheld here. Can you do the exercises with a pencil, or a tablet, or on your iPad? Absolutely. Will you miss out on elements of the lessons? Certainly.
At the end of the day, there will be people who will respond with, "Well I don't like drawing in pen." The fact of the matter is, what you like is either the most important thing in the world, or completely and utterly irrelevant. If you are looking to learn because you think it's a fun hobby and are interested in it for its amusement value alone, then absolutely use whatever tool you like. There is still much to be learned from these lessons regardless of what tools you use. There's absolutely nothing wrong with doing this as a hobby, and no one should ever look down on you for it.
If, however, you wish to learn to draw as a means to an end - to learn to visually communicate so you can move forwards in a particular career path, or just because you are interested in improving as efficiently as you can, then remember this - you don't get to say, "I don't like it." If you haven't tried something enough to fully appreciate its worth, if you haven't conquered a challenge, you are not in a position to claim that the challenge is not worth your effort and time. Once it's overcome, it is your right to denounce it, to say it was a waste. But until then, you're merely avoiding it because it frightens you, or that you worry that you simply can't do it. And on that point, you're wrong.