Note: Although I have never done this, I have looked into it. I have worked a lot with pulse electronics and think the concept of using a spark-plug timing light in this manner is sound.
What kind of pickup does your timing light have? Inductive? Capacitive? I dunno?
Most I've seen are inductive.
What you need is inductive.
The inductive pickup "clothespin" is looking for really quick changes in current flow in the wire ( may be a loop of several turns ) inside the magnetic pole of the sensor. Consider that as the primary winding of a transformer. The secondary winding is connected to the xenon tube trigger circuit of your flash gun via the lead from pickup to gun.
Just to make sure...if you have your light handy, power it up and clip a piece of wire in it.
Short out a AA or so flashlight battery with it. No hard shorts...just quick taps to mimic a short current pulse.
If you can trigger the light with the flashlight battery, well, the output of that piezo adapter is going to magnetically look a lot like what you just did - triggered by the piezo sensor.
The concern I have is polarity. It may make a degree or so difference which direction the current passes through the loop, that is, is it to fire on the leading or trailing edge of the trigger pulse? You may have to determine that by experiment; choose the earliest one.
The pulse may be so narrow the error is so small it can't be seen. Like I say, I have never done it, and only have book learnin' skill.
When one has actually done it, then he gets the street cred.
I don't have that yet.