Is that 1.3V with the camera in circuit, or open? If it's with the camera on the end that means you have more voltage drop in the wire than in the camera, which suggests a very high current flow and something amiss somewhere.
The linked device comes with no suggestion for how it's designed, but probably has de-coupling caps.
https://en.wikipedia.org/wiki/Decoupling_capacitor
Which will block DC and make getting this circuit working a problem. You would want to be using the amp in DC mode, but don't care about things like bias and linearity needed by that audio amp so would be driving it with very high gain as a comparator,
https://en.wikipedia.org/wiki/Comparator
Comparing the Teensy output to say 1.5V and getting a rail to rail swing.
That said all of this is overkill unless trying to get really fast pulses, and most likely a couple of resistors and a transistor will do the job
https://learn.sparkfun.com/tutorials/led-light-bar-hookup where your camera goes in place of the LED
gives one solution but for this a bog standard NPN transistor (whatever you have to hand or can get locally) will do the job with something like a 4K resistor between base and the pin.
Rig it up at first to drive an LED + ~470ohm LED from 5V to make sure it works.
Then you need to find out how your camera actually triggers.
If it reads a voltage to trigger than you just leave the LED in place and wire the camera at the LED/transistor junction which will be swinging 4.5V to close to 0V.
If it does what I'm suspecting it does and it measures the current through it to trigger then you put it in place of the LED/resistor pair with one wire to 5V and the return to the transistor (not ground) and the transistor works as a switch to allow current to flow. If your rig has power supple that doesn't allow this then you need a PNP transistor on the 5V supply and a slightly more complex base circuit to turn it on.
May require a read of the manual for how it is intended to be triggered (current or voltage).