I always assumed there are academic experts on water flows and all the science involved in that- I imagine police would have consulted such experts to understand feasibility/timings etc. based on the relevant natural forces alone- there is certainly some science/relevant calculations that can be made to make a pretty strong working assumption/hypothesis. I would be extremely surprised if the police don’t have such expert analysis/input and the fact they haven’t ruled that out means that potentially the expert advice is suggesting, yes, it is “scientifically” possible that sadly a human can fall into river and not have surfaced/been found yet.
This is a very tricky case as of course, despite being the most likely turn of events, there is no supporting evidence.