Berlin provides a comparison with a coastal city whose sea access can be reopened by ships. A river through the city does not by itself give an outside force a usable supply route. The intervening approaches can remain under the blockading power's control. The logistical point is therefore access along the whole route, beyond the existence of a waterway at the destination.
Notable Point · 39:29-41:03
Why a river does not guarantee relief from a blockade
The Berlin comparison distinguishes having a river from having a supply route that relief forces can use.