If you are talking about the bombs and grenades on page 267 of the DMG then Colossus Slayer does not apply, because the Ranger is not making an attack. No Attack Roll will be made - those in the area of effect will make a Saving Throw to avoid damage.
In general, AoE attacks always involve Saving Throws not Attack Rolls, so Colossus Slayer is never going to come into play with AoE damage. This includes thrown flasks (oil, holy water, acid), caltrops, etc. (Hat-tip SirTechSpec.)
It might come up with an arrow of fireballs or bolt of detonation type item - a piece of missile ammo which hits a target and then explodes. Usually with these items, an Attack Roll determines whether or not the primary target is hit and then a Saving Throw determines whether a nearby target is damaged. In this case, Colossus Slayer would only apply to the initial target, not to the nearby targets since no Attack Roll was made against them. I don't see any such items in the 5E DMG, however.