Right, which is exactly why "The first symptom of stagnation is preference." is dumb. How do you tell the difference between "stagnation" and knowing the best tool for the job? What's that you say? It takes time and thinking beyond a pithy saying? Then why bother with the pithy saying?
Knowing the best tool for the job means that you pick different tools for different jobs. Having a (strong) preference means you pick the same tool if at all possible.