Aluminum can be a great frame material. I have a late 1980s Cannondale from 6061-T6. Newer aluminum bikes use 7005 aluminum as it does not require post-weld heat-treating like 6061-T6.

In general, aluminum frames need to be designed for greater rigidity, as aluminum being flexed a lot is bad for fatigue strength. But for many bikes, like my old race bike, this works out well, with both great rigidity and lower frame weight.

Next, if highly stressed weld areas, "dressing" (filing or sanding) the welds to a smooth concave transition with the tubes, also helps greatly in improving fatigue life, as more "bulbous" (convex) welds will have a stress concentration where they meet the tubes. My Cannondale had wonderfully dressed welds, that was a real calling card of the bike.

Titanium is great stuff, half the weight of steel and much better fatigue resistance than aluminum for the same geometry. But expensive material, and processing.

That said, really good quality (like 4130 chrome-moly) steel is also still a really great frame material, and relatively low cost, however to achieve low weight, by increasing the tube diameter and reducing the tube wall thickness, there reaches a point where that thin a material is really difficult to weld. Double-butted tubes with thicker wall at the ends, have been used for many decades on steel road race bikes, most especially brazed into lugs rather than welded.

Video: When they said "low carbon aluminum", my first thought was, aluminum is already very low carbon content. They meant how it is produced.