Please do not read this as some kind of critique or warning, but.....

Thomson, like many makers of "machined from billet" stems don't do a step that was common BITD of forged quill stems.

Ball burnishing "tightens" up the surface structure of aluminum parts, partly duplicating and/or enhancing the effects of near net forging. By compressing and flowing the surface it closes surface micro cracks that may otherwise become the seeds from which cracks grow.

As mentioned, ball burnishing used to be SOP, but has gone out of fashion, probably for cosmetic reasons. Also keep in mind, that aluminum, (both forged and machined) is vulnerable to corrosion, which cause surface defects that later start the cracking/fatigue process.

On the bright side well designed and built can tolerate crack formation for quite a while before failing. I suspect that this stem's crack woulf have become obvious either by sight or sound (creaking) or even by feeling looser well before it let go.

So, don't obsess, but don't neglect or take your aluminum parts for granted either.