He has no idea how to think at a strategic level and game out what the effects of the flooding would be long term, and was thinking about the dam in very simplistic terms that ignore so many factors; hint, this is not good for Ukraine either, not by a long shot.
Yes, Ukraine had hit one of the flood gates before, and had targeted the road way to hinder Russians bringing armor over it back when they were advancing on Kherson. Also, no one in the area heard incoming shell last night, this was an internal demolition, not a missile or shell strike.
He also ignores that Russia did this likely to make cross bank raids/movements even harder across the river, cut off the Crimean canal to make Crimea a desert when Ukraine reclaims it, and to make the situation at the nuclear plant up-river even dicer, and Russia has already staged weapons and explosives in that nuke plant.
Blowing the dam also is going to leave multiple irrigation canals, besides the Crimean canal, empty for likely years, maybe decades, and those are one's Ukraine needs for it's people.
This is Russia going 'if I cannot have it, no one can' to all of the areas they control between Kherson and the Sea of Azov, because they expect to lose those lands when the Ukrainian's cut the land bridge to Crimea.
This is the same as the Nazi's torching anything the allies could use as they began retreating in Europe, and Russia blew a dam over the same river back in WW2 to delay the Nazi's in 1941, killing ~100k Ukrainians in the process.
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