It tells us nothing for certain. We don't know that every single Springer would go into this stretch of water in every circumstance. And if NB did go into the water, we don't know that Willow saw it happen anyway - she may have been wandering around confused as to where NB was. And the widely-accepted view that Willow was dry so couldn't have gone into the water is based on the evidence of one witness, so we can't even be sure that it's accurate. That's a lot of uncertainty.
At the same time, there is - as yet - no evidence whatsoever that NB was attacked or abducted. So we have three potential scenarios: 1) NB either fell or was pushed into the water and drowned, and her body moved very quickly downstream, which at least one expert has said is highly implausible given depth/flow of the river; 2) NB was abducted in an extremely well-planned and efficient operation that has so far yielded no evidence; or 3) NB was attacked at the scene and hidden elsewhere in a way that has so far yielded no evidence.
I understand why the police are still leaning towards 1) because in the absence of any evidence to the contrary, it remains the most plausible scenario. This could of course change if they discover evidence for one of the other two scenarios, but they don't seem to have found any yet.