The welds would certainly still have low strength if a gusset were used, but the welds would be in an area that had less bending moment, and therefore less stress.
Aluminum weld design can be tricky. Sometimes you will see an aluminum tube that has a short length of telescoping tube just at the end for approximately 1.5 inches or so, and then another weld to join the telescoping part. This is done for the same reason (putting the welds that are at nominal tube diameter in an area away from teh high bending stress).
I have only ever had one welded aluminum component fully solution treated, quenched, and aged after welding. This was a suspension mount for a large unmanned robotic vehicle for which I worked on the chassis design and manufacture.
there is a picture of the vehicle with the suspension mounting bulkhead shown here (vertical item between the middle and rear tires).
Shoptask 3-in-1 (not currently in my garage, but I own it...)
Any day on a motorcycle like this that ends just needing parts and labor is a good day.
4.95, 154mph 1/8th mile 7.49, 194.27mph 1/4 mile