I'm going to check your math on this one right now.>goes to /pol/ and ctrl+f's the word "cuck" for the first ten most-replied to threads.
Thread 1: 8 results (trump related thread)
Thread 2: Five. (trump related thread)
Thread 3: Three results. (trump related thread)
Thread 4: Four Results. (trump related thread)
Thread 5: one result (islam related thread)
Thread 6: fifteen results (The_Donald related thread)
Thread 7: None! Grats. (not trump related)
Thread 8: None! Grats. (not trump related)
Thread 9: None! Grats. (not trump related)
Thread 10: ten results. (trump related)
That's the ten most active threads on /pol/ right now.
So you're right, "cuck" is not really a /pol/ thing. It seems to be more in use by trump supporters. The single mention in the islam thread was a posted with a picture of a donald trump meme, so it's safe to assume that if you're a trump supporter on /pol/ "cuck" is likely your go-to insult.