In your case I would say that you're AB test was successful; however, you didn't go far enough. Once you were getting increased traffic to your signup page, it's time to AB test the signup page and improve that conversion.
This was my thought at first, but thinking about it now, their AB test actually caused a decrease in conversion on the signup page. And that's the point they're making: they drove traffic to the signup page more effectively, but that traffic was no more interested in actually signing up. You can do a lot of things to massage the stats if you're only optimizing for a single step. You can make your homepage promise people a golden pony if they'll only click on the link to your pricing page, but they're not going to get there and suddenly decide they don't care about the pony and want to give you money instead.
This was my thought at first, but thinking about it now, their AB test actually caused a decrease in conversion on the signup page. And that's the point they're making: they drove traffic to the signup page more effectively, but that traffic was no more interested in actually signing up. You can do a lot of things to massage the stats if you're only optimizing for a single step. You can make your homepage promise people a golden pony if they'll only click on the link to your pricing page, but they're not going to get there and suddenly decide they don't care about the pony and want to give you money instead.