Like with everything, there are engineering tolerances.
6061 can have up to 0.15% "Other", as long as no single "Other" component is more than 0.05%. So no more than 0.05% scandium if you want to call it 6061.
This seemed like marketing speak to me so I went looking for research on it. I found something which was interesting:
For instance, the addition of Sc and Zr to 6061 alloy shows a slower ageing response at 190 °C, resulting in lower yield strength of 321 MPa (aged for 5 h) than base 6061 alloys (350 MPa) [64]. On prolonged ageing (36 h), a significant drop (~ 19%) in the yield strength of 6061 alloys was reported compared to only a ~ 6% drop in ScZr-modified 6061 alloys. [1]
I wonder if I am reading this right or if 6061 is actually not very alloyable with scandium.
The paper you linked puts the solubility of Sc around 0.25% in Al. They have some cool TEM work in there looking at how Zn modifies the ordered ScAl3 precipitates that form during aging, which I think is the context for the bit you quoted.
Seems like the main difficulty in these alloys is that the Sc changes the solubility of the copper?
6061 can have up to 0.15% "Other", as long as no single "Other" component is more than 0.05%. So no more than 0.05% scandium if you want to call it 6061.
See https://www.gabrian.com/6061-aluminum-properties/