That article is irrelevant to Opera 15. The quality of the code might be excellent, but sites still didn’t work. And all those features were creating a maintenance nightmare.
It doesn’t acquire bugs just by sitting around on your hard drive? No, but Opera’s code isn’t just sitting around. It’s being developed which means that new regressions appear all the time (just like all actively developed software).
You should not link to an article you don’t understand.