They did not cut the line to Crimea, that is on the east bank and can be supplied without the dam apparently, and they damaged the dam just enough to engulf and drown the islands but not flooding the RF controlled territory. If you ask me, the only thing that does not match is them not shelling the boats that were evacuating the Ukrainian soldiers.
I don't know what they planned, but seems that things didn't went as planned. Some of RF controlled cites and villages like Nova Kahovka or Oleshky become almost completely flooded. Also, floodfing broke first defense line of Russia. And this information was posted by Russian sources.
Also, Russians telling that dam was destroyed by Himars. But you must be really dumb to believe that's possible to destroy such dam and make so much damage with Himars. You only can do it from inside.