That's is pretty much how I understood it. I think 6/4 tubing is available, but it's so expensive that frame builders just use the sheeting and weld it into tubes. 6/4 is suppose to be stiffer, and I guess that makes sense with 3 % more Aluminum, but it seems like it would be a little more susceptible to failure with the weld.
If you're only 50 kg, I wouldn't think you'd need a 6/4 bike. Even a 6/4 won't be as stiff as Aluminum (or harsh that is to say), but at your weight you probably just don't need it.
Now myself, I weight 180 lbs. There may be other factors involved such as wheelset, fork, etc. but I have noticed a difference in flex between a 6/4 bike and a 3/2.5. The 6/4 was much stiffer out of the saddle, and I felt like I was much more efficient. I am not certain of the long term durability of it though :confused: