This will also work for
Enter into a Friendly Free Roam session. First thing you’ll want to do is whistle for your mount (D-pad up). If it doesn’t come, you’re in a glitched lobby, and can proceed with the solution; if it does come, you’ll need to find a new session (Back + Friendly Free Roam).
Glitched lobbies seem very inconsistent; you may start out in a glitched lobby, then at some point it may change from being glitched. This might have something to do with people leaving and entering the lobby. Also, if someone gets too close to the hideout, it seems to prevent you from being able to continuously complete it. A lot of the time if you just wait long enough, the lobby will return to being glitched. Just be aware, if you leave the session, your points will reset.
Now bring up the map and place a waypoint on the Fort Mercer Gang Hideout (directly below Rio Bravo). Proceed to the nearest Transport (fast travel wheel icon on map), access it, and press A to warp to your waypoint.
Once you arrive, you’ll immediately complete the Hideout and receive 400 XP. Press left on the D-pad to replay the Hideout; again, you’ll immediately complete it and receive 400 XP. Now, continue spamming left on the D-pad and you will continue to replay/complete it, each time netting you 400 XP.
Soon enough, you will reach 9001 points and the achievement will pop. Hope this was helpful and happy hunting!