It's the "self-hosting" part that's the "heavy lifting" for MongoDB. They could've also chosen a hosted Mongo service, and that would've worked too. The idea is to spend time focusing on your own product and do as little as possible outside of that core competency. Setting up, optimizing, monitoring, upgrading and everything else that goes into running your own database, however standard, falls outside of that core. The more you can do to avoid doing work that isn't directly building features and experience for your customers, the more you'll be successful.

By undifferentiated, they mean outsource anything that doesn't differentiate their own product offering. Build things that give you a competitive advantage over other products in your space. Outsource everything else because wasting work on it won't help you get customers. Of course DynamoDB is differentiated for Amazon, but it's undifferentiated for Intercom...it's just a persistence strategy that customers should never know about.