Actually, you can cook both sirloin and eye of round to falling apart tender and cook it that long. They do not need to be cooked only to rare. The same rules that apply to brisket or chuck, apply to any beef. Cook it long enough, the protein will break down and the meat will fall apart. I might argue that it is not the best way to cook sirloin, but, it can be done.
The key is that you do need to have some moisture in the pan, as this is what prevents it from completely drying out. And you must have a covered cooking vessel. The traditional roast done this way is also roasted with fruit, often prunes or stone fruit. I prefer red wine, butter and water to Liptons, but, to each their own. Smoke for 30 minutes, then place the roast in a covered casserole or braising pan, add aromatics, fruit and liquid, maybe 2 cups, then roast for three hours, at 425F. Then turn oven off, do not open door. Allow to cook for another 2 hours. It should be fork tender. Of course, meat varies, but, this works.
me: I don't drink anymore
Yelonutz: me either, but, then again, I don't drink any less