He's stated before he likes the employee-free lifestyle. And there is a huge gap to bridge between "I can do this myself" and "There's enough work to hire a full-time employee".
The amount of work is, I suspect, not a factor. Im not sure that having an employee wouldn't be a move I'd ever want. Yes, that means I'll never pass a certain level of success by the measure the majority use (money). However if that's the price, so be it.
But getting one or more partners on board who get like 10% of the revenue and take over all the development work and day to day business might have worked quite well. Or subcontracting to some kick-ass freelancers (who would have jumped at that opportunity, surely?).
Because he obviously loves to code, and once you have employees you become a manager. I'm looking forward to see the things he's planning to build.
Good luck Marco!
The other complication is that Instapaper is in a weird place in its lifecycle. It's got lots of paid users, but they paid years ago, and now they are just a drag on making any kind of meaningful changes. Investing a lot of money to improve the app probably wouldn't pay off.