The stability triangle primarily applies to counterbalanced trucks. Reach trucks capacilty is based on straddle support this is why the higher you lift the wider the outrigger arms need to be. When the reach mechanism is retracted the CG of the load is positioned inside the area of stradddle support making the unit more stable.
I will agree that reach trucks tip overs will be to the side -generally when a "cowboy" pulls a load out with the mast and reach extended at the acceleration rate of an NASCAR driver. This is one reason why reach truck travel speeds are reduced at extended heights.