Sunnianswer: This silly claim is based on translating the term "jawhar" used in belief science as equivalent to the term "atom" in physics. Belief science in Islam is not physics and it is a mistake to confuse the two. The jawhar in belief science is the term for the indivisible particle that bodies are made of. It is not important in belief what particle is the indivisible one, this is the concern of physics. Is it the quark, for example, or something else?
The scholars said that if bodies are divided into smaller and smaller parts, one will eventually reach a particle that is impossible to divide in the mind’s eye. Some of the Greek philosophers denied this, because it disagreed with their idea that the world is eternal.
See The Indivisible Element.