If this is used on a rain team, Choice band Zekrom outspeeds and 2HKOs so this is not a counter. What does this do anyway? Giratina takes next to nothing
Specially Defensive Giratina which is the most common set on sand teams takes 47.1% - 55.5% from CB Earthquake. Does not look like a small hit to me. Even physically Defensive Giratina takes 35.2% - 41.4%. If you have Spikes and Stealth Rock support, you might 2HKO it. Also, Excadrill should run at least 217 Speed, underspeeding Giratina is dumb.
Excadrill does not counter Zekrom but it checks it. CB Earthquake OHKOes Max HP Zekrom easily. Obviously, Excadrill should use more Speed.
Don't get me wrong, SD Excadrill is still superior to CB in almost every case.