Please double check that in case I'm missing something obvious. I searched for errata but couldn't find any and swift poisoning (the replaced ability) is also gained at level 6.
Directed Blast (Su): At 6th level, a grenadier can detonate a bomb so that it splashes in a 20-foot cone rather than affecting a radius. The cone starts at the alchemist and extends away from her in the direction she chooses. The alchemist designates one creature in the squares affected by the cone to be the target of the bomb and makes her attack roll against that creature; all other squares in the cone take splash damage. If the alchemist has the explosive bomb discovery and throws an explosive directed blast, the cone of splash damage is 30 feet long instead of 20 feet. This ability replaces swift poisoning.