単なる (tannaru) means “mere” | MotiTan Japanese Dictionary

More examples

その話は単なる噂にすぎません。so no hanashi wa tan na ru uwasa ni su gi ma se n

That story is nothing more than a rumor.

It marks something as nothing more than the noun it modifies, often limiting its importance or scope.

ほんのhonno: A small amount or degree of something, often presented as unimportant or insufficient.

ほかの意味や価値を加えず、ただそれだけであるさま。ho ka no i mi ya ka chi o kuwa e zu ta da so re da ke de a ru sa ma

単なる (tannaru) combines 単 (tan, "single") with なる (naru), the attributive form of the classical copula なり (nari, "to be").

単なる (tannaru) can sound neutral in factual descriptions but dismissive when it reduces a person's achievement or a serious issue to something ordinary.