Disclaimer: I don't own How I Met Your Mother and never will. No copyright infringement intended.
Barney never meant to pour his heart out like he did at Splitsville but once he got going, he couldn't stop. He meant every word he said to Nick about his feelings for Robin. He's always loved her – even when he didn't know it – and he always will. Even the pain and rejection has been worth it as long as he gets to see her every day.
What Barney felt for Shannon doesn't even compare to what he feels for Robin now. He literally cannot imagine his life without her – She is his soul mate, after all.
But the fear of another rejection, of ruining their friendship, holds him back, which is why he's secretly grateful when Patrice calls. He can't handle going through the heartbreak of last November again. And that is why Barney plays it off as being a bro for her, and hopes she buys it.
Robin doesn't buy it, but she also doesn't call Barney out on it. She's terrified of going down that road again because that fear of it ending badly again weighs on her mind. She'd rather keep him as a friend than lose him forever because they couldn't make their relationship work again.
What he said was one of the sweetest – and most honest – things she's ever heard; especially coming out of Barney's mouth, but Robin isn't sure how to deal with what she said. Ted loved her, but Robin doesn't think he loved her as much as Barney clearly does. How can he be that much in love with her? She doesn't understand.
So Robin lets Barney get away with his lies about being a bro for her because it's easier and she's secretly grateful when Patrice interrupts their almost kiss.
They'll get there soon, but now isn't the right time.