Generally true, but sometimes not … and you won’t know until you’ve tried combining things! Two years ago I sold a story to Asimov’s (“Leaving the Station”) that started out as two separate stories. It was the collision between them that set off sparks.
In IF, a couple of times I’ve had puzzles from an abandoned work that found their way into a game that I completed.
Never throw away your notes on any game or story idea! If it isn’t working, set it aside and maybe come back to it later. (I have 30-year-old notebooks in the garage that I really ought to run through the shredder before I die…)
Right now I’m developing an idea for a game that I first looked at at least 3 years ago. I couldn’t figure out what to do with it – it went wandering through a lot of tangled bits and tied itself in knots. Now suddenly it’s making a lot of sense, because I turned it sideways and pruned away some of the accumulated deadwood.
My motto has always been: Ideas are cheap and plentiful. What’s neither cheap nor plentiful is the time and energy (and the insight) with which to develop them into completed works.