That's true enough, but I would add something. By David Gerrold's own account, he read every sci-fi novel he could get his hands on as a teenager, checking out the weekly limit on his library card.
I think he read THE ROLLING STONES, forgot it, and then his subconscious regurgitated it when it was time to think up a story. Same deal with Gene Coon writing "Arena" without realizing that he had read the Brown story.
Everything we create is influenced by our past experience to some degree. That's just how it works -- fiction is a conversation between past and present writers, as the tellers of new stories react to, reinterpret, homage, or deconstruct ideas that they've read in other author's works. There is simply no such thing as a work of fiction that isn't influenced by earlier works of fiction. It's just a question of whether the influence of a single specific work is so dominant that you're essentially telling a version of the same story. Heinlein himself didn't think that "Tribbles" bore enough common elements to his work to count as an imitation or adaptation, and who'd know better than he?