If there are parts, there is also a whole. A whole is not the same as parts. So if you mean by “reductionism” that there are only parts and no wholes, then reductionism is false.
If you mean by reductionism that a thing is made of its parts rather than made of its parts plus one other part, then reductionism is true: a whole is made out of its parts, not of the parts plus another part (which would be redundant and absurd.). But it is made “out of” it—it is not the same as the parts.
If there are parts, there is also a whole. A whole is not the same as parts. So if you mean by “reductionism” that there are only parts and no wholes, then reductionism is false.
If you mean by reductionism that a thing is made of its parts rather than made of its parts plus one other part, then reductionism is true: a whole is made out of its parts, not of the parts plus another part (which would be redundant and absurd.). But it is made “out of” it—it is not the same as the parts.