It really depends on the guys preferences. Some guys are attracted to it and some guys may find it repulsive. The guy who you are referring to is obviously attracted to it. Personally I would be too because I like girls who can be themselves in front of me, as long as they have manners and remember to say 'excuse me'.
I think it goes much deeper than a simple "burp". Burping is considered rude, and it challenges the mindset that all girls are or should always have perfect manners and not be rude, while it is considered more acceptable for guys to be rude. Thus why it may seem sexy for a girl to burp, to some guys, is that is shows that they challenge the view and can relate to guys (anybody likes someone who relates to them/ their "kind"), or just don't care and are free to be themselves and free from mindsets placed upon gender.
Burping in itself is not considered sexy and should probably not. I believe everybody should have good manners no matter their race, culture, age or gender.
To be completely honest, and it'll blow your mind, some guys are turned on by girls belching, in the same way that others are turned on by feet or balloons or bubble wrap or whatever else. There's a lot of weird fetishes out there, and a burping fetish is indeed rare, but one of them. Feel free to google it. I speak from experience, and though there are many explanations out there as to why, I think the best one is as follows: Just because. So is it sexy? No for most people, yes for a few, a moderate amount indifferent. Why? To each his own.