Ned, it’s a generally a good bag. Don’t think it puts that much strain on the seat post but I don’t know if I’d use it on a carbon jobby. It uses tight metal bands to hold on. Doesn’t move around too much at all. I have mine hard up underneath my saddle to help prevent spinning. A bit of inner tube rubber around the seat post helps too.
You might be better off SQR Tour for off road use. It’s deeper, not quite as wide and had side pockets for tools.
Niggles? A couple. Biggest one is that there exposed bolt ends inside the bag that can scratch you (or clothes, laptop etc). I just covered them up by gluing old inner tubes over them. Second is that the rear handle tends to slip down over the rear reflective patch (or light if you fit one). I just ziptied it up. And it can be a little rattley if it isn’t full (it’s a stuff outer and has no compression straps). I just fill mine.
Good bag all in though. Feels like it will last too.