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 precipitat

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? For context for those not familiar, precipitation hardening (aging) is the principal way to increase strength in Al alloys since you can’t quench harden them https://en.m.wikipedia.org/wiki/Precipitation_hardening https://en.m.wikipedia.org/wiki/Precipitation_hardening