cos even tho it's slightly insoluble what does dissolve ionises completely 100% into Ca2+ and OH- ions, it just accounts for how much of what can be dissolved is ionisable. 100% ionisable doesn't have to be 100% dissolvable
You are using the Arrhenius theory of a strong base. The Brönsted-Lowry theory is not restricted to aqueous solutions. If Ca(OH)2 is a good proton acceptor (which it is), then it is a strong base according to the latter theory.