Yes, absolutely. Our app uses the Facebook Marketing API to query interest suggestions. That is fully approved usage of the API's functionality. When you want to apply interests straight into your existing Facebook ad sets, you can connect your Facebook account to InterestExplorer and approve the required permission. InterestExplorer is fully approved by Facebook to ask for this permission and perform this action.
Facebook wants developers to use their technology to build value-adding services like we did. This is not a loop-hole or a hack that will eventually be shut down by Facebook, that's why they neatly document how it should be used in their developer documentation.