For how it’s mounted, see here.
It’s not ideal as the rack slopes downwards quite a bit at the back. As I mention in that post, the bag’s slope stops it looking too bad but anything heavy that you put in it ends up right at the back unless you pack it in with clothing, which isn’t ideal.
A lot of people seem to now use these.
As to what bag it is, I can’t actually remember at the moment but it started falling apart quite quickly so I’d not recommend it. Any small rack mountable bag should work well.
Edit: Apologies, I should have read the actual post you were referencing.
It’s a Vaude top tube bag like this.
It’s mounted on the frame/seatpost and the bottom of my T-bar. As I don’t have the bars out very far and I never cut it down, it gives me that ‘top tube’ to attach to. I also have my front light battery attached on the top there as you can see in the picture.