Here's an example of something unimportant: maybe Microsoft C, version 7, which was released in 1992, has a typo in a menu item that was never fixed. (The next version of the software was Microsoft Visual C++ version 1.0). True, nobody has used Microsoft C version 7 to compile a program in many years now, but - there you go. An unimportant problem.
That would be an example of an unimportant problem. Releasing a patch to fix the menu typo right in the binary would be an unimportant solution to an unimportant problem. Maybe if you take the time to release it, literally 0 people ever apply that patch from now until the end of time.
If you want to work on an "unimportant problem" fix bugs in software versions that were discontinued and which nobody used because of a quick migration to the next branch. Work on fixing Windows 2, maybe.
No, what the author really means is work on something very important, that doesn't evaluate as such to everyone, (or to the people you normally pay attention to). That, I can agree with.