The Signaling Line Circuit (SLC) is meant to carry signals between addressable devices (Modules and Detectors) and the panel.
In some ways, the addressable devices can be thought of as extensions of the panel itself. The SLC can be thought of as the data/power buss connecting the modules to the CPU in the panel. Power to run the modules and detectors is a secondary purpose of the SLC; the only purpose of the power that rides on the SLC (data bus) is to power the modules and detectors.
Because the power (electrical current) is so low, the devices on the buss can manipulate the current to produce huge voltage swings in the data that is sent back and forth. This makes sending the data between the devices and the panel very reliable.
On the other hand, the power to run the horns on the Notification Appliance Circuit (NAC), the strobes, chimes, bells, or other notification devices, is huge; the current isn't measured in micro-amps, it is measured in amps.
If the SLC powered a bunch of strobes, the SLC would have to be able to carry hundreds to thousands of times more current than it normally does now.
Because of the high current of the horns and strobes, when sending data to and from the panel, the devices themselves wouldn't be able to affect the voltage very much. The data sent by the devices would not be as reliable.
Also, as a result of the higher current draw of the notification appliances on the circuit, the wires themselves have to be much larger.
True, a few manufactures have put data on the NAC to make the horns and strobes addressable. However, unlike the SLC, the distances for the NAC can only be a few hundred feet.
What all this means is no, the SLC cannot power a horn/strobe, and yes, a separate pair of wires has to be used to carry power to the notification appliances (horns and strobes).