Not sure what proper HN etiquette on this is, but I made a comment on the launch news story on this exact subject:

https://news.ycombinator.com/item?id=8525057

TLDR- Those engines may be 50 years old, but they're a technological marvel. Using them is a little crazy, but I'm sure Orbital saw it as the "least crazy" of three options: building a new engine from scratch, buying prohibitively expensive engines from Rocketdyne, or buying risky but cheap engines from Russia.