Perhaps the word is modern, but cults have been around forever. Just identifying them in a different way is still appropriate. There are distinct attributes of cults, versus, say, just an unbeliever who isn’t involved with any particular groups at all.
The bible already has a perfectly good word for derived, surface imitations of Christianity that differ deeply enough to matter but are similar enough to be dangerous temptations: heretic.
The official LDS, and Christian Science would qualify. Shinto and Hindu congregations would not.
Scientology would be a wrong religion that would qualify by most modern definitions as a cult, but NOT as heretic. It was never meant to be fake Christianity.