(not 100% sure if this might be intended)
Decoy does not emit a sonic wave with Kaelthan Blasting agent and 3 points skilled into short fuse to further reduce the duration (it does not apply frailty/shred to enemies)
I guess this is because it does not exist long enough (without Kelthan Blasting Agent equipped Sonic Wave does work) and (afaik) no visual model of the decoy is created because of the short duration.
However the passive Sonic Detonation does read “The Decoy emits a Sonic Wave when it appears” and for the Decoy to theoretically detonate at all it must have somehow “appeared” first (even if it is just for a split second).