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.
Yep, a dam is damm difficult to blow up and certainly using a HIMARS is like using a spoon to kill a bull. However. there are tools that can damage it enough from the outside, namely some GBU bombs that US has developed pretty much for that type of situation (like massively protected underground bunkers and the like).
Ukraine does not have any of these though... If I have to bet, this was RF.
TBH I think that the RF army cares zero for the villages. The claims that it destroyed the first line of defence does not seem realistic. I would have to see what the sources say over the next few days on regards to what was used and what effects happened.