The middle of a downtube is , relatively speaking, a low stress area. I wouldn't trust that weld on the spez, if it were, say, a swingarm joint. Not so much because of the quality of the weld, but moreso for the lack of heat treating after the weld was done.
All production aluminum frames are welded up, then baked at 800+ degrees for heat treating, oil quenched and re-aligned, before being painted or re-heated for a second round of treatment. Aluminum is just so tempermental post-welding. That's why I'm generally skeptical of DIY or even factory aluminum frame repairs at joints.
My next frame is probably going to be steel.