Is it right that super old whiskey and cognac ( like 30 years and more)taste the same?

For spirit that has had 30 years of barrel aging, the barrel dominates the flavor profile. They won't taste the same in any case because the aging regimen and the barrels used will be different. The cognac will also still have some of its wineyness, and some whiskys (strongly peated scotch, for example) will also retain some distinctive character.