I agree that "work" should highly correlate to fun, under normal circumstances. Still, there is a lot of drudgery on the way to accomplishing goals that needs to be overcome, and there are circumstances where fun might just not be appropriate.

OP gives one example:

You may experience flow states once your attention is focused where it should be, but unless you have the incredible and rare fortune to have what is shiny match up with what is useful, the act of starting and maintaining focus and improving your ability to do so will be hard work.

Others that come to mind:

• Running your own business is fun, having to fire people is not.

• Exercise is fun once you get into the rhythm, but I had to trick myself into the gym for the first couple of month.

• Having kids can be fun, changing diapers is not

Some possibly rational actions cannot ever be fun

• Killing people is not fun, unless you have psychopathic tendencies

If all you mean is that the post overemphasized the necessity of occasional pain, I agree, and upvoted thusly