Circulating tumor cells, once they enter the bloodstream, have about 2-3 hours to find a place to latch onto (such as the lungs, liver, brain) and start to grow.

When the CTC count goes down, that means there were fewer cancer cells found per the tested amount of blood, which means less chance that the cancer can find a new home somewhere else in the body.

As for correcting someone, if someone states something as a fact and it is clearly wrong (not wrong based on opinion), they should be corrected so they do not spread misinformation. That is why we know that the earth is flat and the moon landing was fake. Oh, wait...