That sounds like it is probably true. (Riff Raf and Cleo were DIC's own characters, and shouldn't be in the erd party owned HEathcliff.) Stuff like that generally does influence those decisions.
Besides, what would Heathcliff and Riff Raff do in the same episode? The premise was that the the three cats were the stooges, and in both Heathcliff and Riff Raff cartoons, they treated them pretty much the same (Hated that); i.e Heathcliff and Riff Raff pretty much filled the same role.
So that sounds like what the article said: that the 3 cats were specifically designed to be in both series.