The tubing would have initially been mandrel formed, or hydroformed to shape and thickness, thin in the middle, thick at the ends for welding (thus the bend away from the head tube.

The place that bent was thinner.

The tubes may have been left annealed after forming, or heat treated for strength.

Many of the alloys such as 6061 are weldable, with moderate reduction of strength.

The strength can be regained with heat treatment. I can't say everyone working with aluminum heat treats, but many apparently do.

Someone doing repairs likely could replicate the heat treating cycle, but that would also destroy 100% of the paint, and be an additional, fairly specialized step.