It would seem that the word treasure here is being used to modify one's attention. Historically in books about "knowledge work" or commonplacing, the word was used with respect to one's storehouse of knowledge itself and not one's attention. Some of the effect is the result of the break in historical tradition being passed down from one generation to another. It's also an indication that the shift in value has moved not from what one knows or has, but that the attention itself is more valued now, even in a book about excerpting, thinking, and keeping knowledge!

Oh how far we have fallen!

It's also an indication of the extremes of information overload we're facing that the treasure is attention and not the small tidbits of knowledge and understanding we're able to glean from the massive volumes we face on a daily basis.

We can imagine mining the information we encounter, following veins and seams underground, then smelting and refining the ore into useful metals. Occasionally we might come across gems that are nearly perfect when we discover them, perhaps needing only a bit of cutting and setting to reveal their beauty. But mostly the work involves patience and effort, as we go through the steps of finding, collecting, refining, and concentrating information from a raw material into exactly what we need for our structure

I love these two new clever metaphors (mining and refining and cooking) for note taking for building knowledge. They're a welcome addition to the older and more classical metaphor of bees (Latin: apes) collecting pollen to make honey in the commonplace tradition.

He uses the phrase "thoughts as treasures", but I'm reasonably sure that he doesn't do so with the same intention as the centuries of rhetoricians which preceded him.