The proper response to that realization is to unwind the tension, not to keep pushing. Really, he just got lucky, but that's fine, it's an interesting video. I take risks myself when building stuff but I would not trust my life to something that isn't engineered for strength and in spite of all of the work that went into the computations of how to get the projectile up to speed as well as the strength of the arm there is already some proof that he misjudged how strong wood is for this application (one of the arms desintegrated).

Wood is super strong, if used properly, this looks like a setup similar to the neck of a base guitar but without the engineering of the joint to go with it, and a much nastier angle between the direction of pull and the joint.

Safety glassed would have been a good idea, any rubber can suddenly fail. But he is absolutely right that "creaking noises" are common in wood far far far below their breaking point. I suspect he calculated the strength of all these things if he went to that much trouble for the sling. Also the markings on the bolt heads showed they were grade 8 (those 6 radial lines) which are near the best common off-the-shelf bolt strengths you can get. The OG wooden sling breaking was likely a nuanced issue stemming from it's rotational speed, so not a risk during the cranking stage. The sling wasn't even under load at that point. High-speed rotational effects are many orders of magnitude harder to predict than the needed strength of a base structure. But yes, safety glasses would have been wiser.

I don't think it's the bolts shearing we should be concerned about. Rather, the wooden beams might be split in the middle by the torque on the four bolts.

This guy seems like he's probably a very qualified engineer, yes he should be wearing some safety equipment but he also seems like he knows how close he is to the limits of the device far far more accurately than the shade tree maker.

He may very well have designed in a 50% or more safety margin on the frame.

He spent all that time engineering the speed, he forgot to engineer the base.