Basically, Chrome allowed users to use the "Always open files of this type" option with executable files. So if anyone was ever foolish enough to set that option after downloading a `.exe` on Windows, any future site they visited could take over their machine just by initiating a download for a malicious executable.
If instead of just complaining that commenter had taken the time to fill out a bug report they could have easily gotten the bounty instead.
Sometimes it just takes a tiny bit of extra effort to go from noticing something's amiss to actually doing something to get it fixed.