Fortunately for us, this one isn't too likely, because realistically, an alien civilization capable of travelling the relevant distance and destroying another civilization isn't something that can be hidden from.

I mean its entirely dependent on whatever theoretical sci-fi gimmick utilized to close that gap. Are we betting on FTL, near the speed of light, or the left field entry ... intra dimensional travel?

The dark Forrest theory is mostly dependent on FTL, where the ability to destroy a planet is on par with the discovery of the planet. Meaning that it's not so much a seek and destroy scenario, but more like two scared drunks stumbling in the dark with loaded shot guns.

They should be able, fairly easily, to examine every planet in the galaxy and see which ones have life on them, and wipe it out before any civilization ever arises at all.

Again, this theory isn't supposing that there is a omnipresent alien race, but that all species are searching in the dark with a flashlight. Just because you have the ability to look everywhere, doesn't mean that you can look everywhere at once, and the universe is infinite.

The fact that we exist at all necessarily implies that nobody in this galaxy has been committed to going this, at least for the past billion years or so.

Again, this presumes that just because you have FTL tech means you have limitless resource and man power. When in reality the theory presupposes that FTL increases resource competition, not diminishes it.