What he means when he says you are earning income when you are not working is running a business that can make money 24/7 even though you are not actively working all the time eg still take orders whilst you are sleeping. Take a look at Pokerstrategy it is making money 24 hours a day 365 days a year, even if they closed their offices right now and sacked everyone they would still be making money 24/7 for years although it would slowly decrease.
That is why this article isn't even "defending" poker most of the points he makes are arguing against poker. Poker is the worst job you can have in regards to this point. You only make money when you are playing, if you are sick, dont play, retire you are out of luck there are no benefits waiting for you.
Poker is a "gamble" in the short and long term. Short term for obvious reasons and in the long term because the environment in which we play is constantly changing. Players that were crushing 5 years ago aren't able to do so now. There is always news of new legislation that may prevent certain people from playing poker. Sure you can win a good amount of money now but what about in 10, 20, 30 years? If the day ever comes when you can't win or you are so sick of poker you want something else what then? Finding a normal job is going to be extemely difficult so you will probably have to be self employed ie your options are limited.
Poker is by far one of the biggest gambles out there and it's no surprise alot of successful players are always looking for alternatives to making money and its mostly the micro/newbies who look at it as a dream job.