Part of your confusion is probably the fact that "London Broil" isn't a cut of meat (it's a recipe/technique) although a lot of stores sell different cuts of beef labelled as "London Broil". There's no telling what cut you're getting, it could be top sirloin, flank, top round, etc.
So what one person says works for London Broil might actually be using a different cut of beef than what you have. If it's flank steak, I just do a quick cook (450-500 x 4-5 min per side). If it's a cut from up on the shoulder, it's going to take longer to tenderize so I'd do a slow roast or reverse sear type of cook.
For best reverse sear results, I like to cook tri-tip up to 5 degrees less than my desired final internal temperature. Then I'll let that rest, uncovered, on a raised rack so the tri tip isn't resting on a flat counter or plate (doing it on a flat surface kind of steams the surface of the meat touching the plate, causing more juices to flow out).
For me, the key is letting the meat rest until the internal temperature stops rising. Then I'll put it back on to sear for just 1-2 minutes per side and the temp rise is negligible at that point.