Chesh, no, lymphocystis can affect all types of fish, including saltwater. There are
different types of iridoviruses that do affect only certain kinds of fish though but generally, those don't cause raised bumps. There's a dwarf gourami iridovirus, an angelfish iridovirus, even one that affects rams. There may even be an anabantoid iridovirus.
Cornelius's age may have caused his immune system to be a little more compromised and that's why he became susceptible to the virus.