No, it's not currently possible to automatically send direct messages (DMs) to people who comment on a Facebook post without using a third-party app. Facebook does not provide a built-in feature for automatically sending DMs based on post comments.
Using third-party apps or services to automate DMs can also be against Facebook's policies, as they prohibit the use of automated systems to send messages on behalf of a user.
If you want to engage with people who comment on your Facebook posts, it's recommended to do so manually and authentically, rather than relying on automated systems. This will help build stronger relationships with your followers and create a more positive experience for everyone on the platform.