Briscoe has made a reasonable speculation based on knowledge of the Arizona Strip in general, its history and its geology.
No reasonable person would take his statement in any other way until it is supported by specific results from scientific investigation of the North Pipes claims. You are correct about that.
However, given everything I know about JB, I take his speculation, i.e. educated guess, seriously and will not be surprised to discover that people capable of taking action are doing the same.