For a dating website retention is extremely important. After all, if the dating website is to be a commercial success the last thing you want with a subscription model is to have everybody use the site once on their trial period and then cancel. Successful dating websites all bank on being able to extend their customer life-cycle as long as possible.
If your retention is good enough (> 98% month-to-month) then you can focus on other things but if it isn't then you're likely going to burn through a lot of cash faking growth when you really should be focusing on why people are leaving and fixing that first before you spend the big marketing bucks.
All other things being equal, a plumber or mechanic isn't better because his/her customers are returning every 3 months instead of every few years.
Likewise, I'd rather use a dating website to meet a long-term partner (and cancel my subscription) than go on a bunch of unsuccessful dates leading to short term relationships (keeping my account open).
A dating website that is delivering what customers want /should/ have low retention. A dating website with excellent retention has probably decided retention is more important than getting their users into long-term relationships.
In the context of the article, all things aren't equal because:
new business != startup
Here's an interesting post about being a successful dating site: http://caseysoftware.com/blog/working-for-a-dating-website
Personally I feel focussing on growth is focusing an all pirate metrics, but one or two at a time. You shouldn't solely focus on acquisition or activation. Especially if your retention is too low. But sometimes your retention is good enough so you can focus on other things.