The flat stock is only potentially "flimsy" under compression - the side the boom is leaning TOWARDS.
However, it is ultra-strong in extension, or, the side the boom is leaning AWAY from. This MORE than counteracts the compression side, because (through triangulation) it keeps that side from compressing in the first place, because it keeps the boom from leaning away to start with.
Your bolt didn't break because of the weight of the engine on the end of the boom. It broke because the boom tilted sideways, due to the lack of bracing. There may have been other issues, but the lack of bracing exacerbated any other smaller issues, and compounded them until failure.
Install the braces. They're there for a reason. And, extend the damn legs when you put a load on. Your lift point should NEVER be outside the ground footprint.
Gravity is a LAW, not a suggestion. Same with physics.