That is right. We leverage some of the AGPL MongoDB code for the parser, as indicated by the License. Our own code can be licensed differently, see a previous discussion on Hacker News [1]. The Mongo API are reasonably stable over the last few years, only seeing very minor changes. Most of the later versions improved upon performance and transactions, which we support natively with our underlying technologies. Still, if you have any specific API that you feel is needed, we'd be happy to implement and we welcome community contributions.
[1] https://news.ycombinator.com/item?id=44937978