The thing is that it correlates proportionally and if you change from a more verbose language to a less verbose language you only change the proportion, not the amount of bug.
It's not really an "incredible sounding statement", it's just logic. What makes a program more bug prone is the amount of logic there is in it. If it takes you 1000 lines of code in language X and it takes you 500 lines of code in language Y to write the exact same thing, you still have the same amount of logic in both language. The only difference will be the density of the code. Changing the density of the code doesn't change the fact that it will have the same amount of potential bug in it.