It was basically the Japanese equivalent of WAP before smartphones but it's completely irrelevant now and even when it was a thing, I'm not sure it was relevant to desktop sites like this article is talking about.
My understanding that i-mode markup was very limited and didn't include tables or css, so I don't think it would be possible for desktop sites from 2011 to have been designed for i-mode first. Common Japanese sites for desktop browsers from this period as described in the original article were extremely cluttered in a way that would have been completely impossible to read on a feature phone with i-mode.
I believe that as with WAP, it was necessary to have completely separate i-mode sites, so I don't think the front end rendering of one would necessarily affect the other even if companies like Yahoo Japan were serving similar content over both.
> Japan was using their version of the mobile web on advanced flip phones long before the iPhone came along and in even larger numbers than had personal computers.
I think the bigger problem is that this is a seven year old article and the web landscape in Japan has changed significantly since then.
Any answer to this question that doesn't include a mention of this is grossly misinformed to the point of being wrong.