This is pretty light on details and it kind of makes no sense. Everyone loves the product and it's a (apparently very) profitable business. What can Andreesen Horowitz do to help them other than come in and do another bubbly ol' flipparoo?
In addition, if founders want to take cash off the table and that is what it takes to make a great investment in github you do it.
A VC wants 10x, I see no better way to allow this than to let a founder take some money off the table and shoot for the moon.
As far as risk is concerned with expansion, as long as the founders and the VC want the same outcome I dont see what the big deal is with taking on more risk, particularly for founders with other peoples money. Even better after theyve taken money off the table!