The hypothesis could be correct, but it would require either the evolution of collar cells in parallel (conversantly) with those of choanoflagellates or for the sponges, having evolved from the choanoflagellates, to not be animals. That second option would mean that the simplest animals are something like the cnidarians.
Work Step by Step
Consider the hypothesis and the fact that there is some evidence to support it. Then try to imagine what would be needed for the sponges to have evolved if that hypothesis is true: another way to have the choanocytes or a way to avoid the sponges and their choanocytes being part of the animals.