Yeah. You put an enormous stress concentration in a high stress area,* didn’t do anything to make up for it, and probably rode it like a proper mountain bike ought to be - of course it failed. Oh well, at least you weren’t (physically) hurt by the failure and can count this as a learning experience. Another data point for Peterson’s stress concentration factors.
IMO this is the way. Braze small cable guides somewhere away from the bottom or top of the tube and you’ll have minimal stress concentration. If you must route the cable internally, reinforce the area with a mating tube on the inside (Devlin) or outside (wzrd).
*Yeah, it’s centered on the neutral axis, but the point stands. It’s on the down tube, which is the largest tube on the bike for a reason.