Well you'd probably want to go deeper than 5 miles to be sure. But a massive comet strike doesn't guarantee the sterilization of ejected material which is really the difficult part. Because there is life that can handle vacuum you have to not only heat the world to 200C (we think) but you have do it in a way that nothing larger than individuals molecules escape due to heating. which is also tricky because at some altitude in the atmosphere evaporative cooling might allow for something to survive indefinite periods of superheated crust.

EDIT: really what it comes down to is how many 9s of biocide do you want. Ramming Luna into the Earth might accomplish 7 or 8 9s, but if you want to kill everything, 100% of life from LEO down not just practically everything it's going to take more finesse than that.