They call it a Catholic belief because they do not believe it is a biblical teaching, and that the doctrine of the Trinity did not appear in formal wording until the Catholic Council of Nicaea (the first one in 325 A.D.) centuries before Protestantism emerged.
Of course, if it is biblical teaching, then it is Christian teaching, irrespective of when Catholicism arose and irrespective of anything it decreed about the doctrine. But JWs have swallowed the half-truths their leaders have told them about the history of the Trinity doctrine and so think it's only a later Catholic belief that has no biblical foundation.
JWs are notoriously ignorant about the history of Christianity from the 2nd century onwards. If they had basic knowledge, they would know that Jesus was being worshipped as God right from His resurrection onwards, and that for Jesus to say the only unforgivable sin was blasphemy against the Holy Spirit was to confirm the status of deity for the Holy Spirit. They would know that trinitarian language (in Latin) was being used by the likes of Tertullian (circa 150-225) in the 2nd century. This proves that the trinity belief was on the go nearly 150 years before Arius attacked it so much that his heresy had to be dealt with in a formal trinitarian statement. When that was formed (325), that was the culmination of debate, not the start of something new, as the JWs claim.
All of this ignorance contributes towards JWs perpetuating the untruth that the Trinity doctrine was a 4th century Catholic teaching. Catholics, Orthodox and Protestants alike agree on the Trinity doctrine, and share the same biblical conviction as to it being soundly Christian, even from the mid 1st century onwards.