First of all, the tweet doesn't say they will cross the reservoir, the tweet actually said the Ukrainian will have a shallower river crossing when the water dried up, water is going to dry up regardless of the dam being blown because it is about to hit summer over there. This is what he meant; this is not about blowing the dam. With the dam in place water is still going to dry up because it is summer and that dam would have prevented the water from flowing downstream to begin with, that's what dam do.....
Second, what that mean actually did not benefit the Ukrainian, because as said, they would have a easier crossing in a week and a half (More likely 2 to 3 weeks) which means you are giving Russian more time to prepare for the crossing if they really do cross there because you are giving them about 2 weeks more to prepare for it. How does it benefit the Ukrainian??