Mike the drain holes are only about 5mm in diameter. There is no way anything is going to get inside from the outside.
Grommets would be useless as they would inevitably mean there would be a lip on them, meaning the water wouldn't drain completely ( the drain holes need to be at the lowest point of the boot.
TBH I haven't checked them for a bit, but hopefully the touch up paint I used to seal the exposed metal is protecting it. Even if it's not, it's still better than a boot full of water which will slowly rust the hole boot rather than two small holes.