The EMP is generated by the lamp/flashing circuit, which is essentially acting like a small (but not atomic) dipole antenna excited by a pulse. Therefore it has a distinctive cut-off frequency, probably in the 10s of Mhz range but I'm not sure exactly (something like this: [1]). This pulse is picked up by wires that act as antenna, and you get RF interference.
The Xenon flash has also a distinct spectrum, but it's essentially contained in the visible light range (in a log-frequency spectral density plot) [2], which is what you want with a lamp! This emission is absorbed by the semiconductor material by exciting bandgaps: displacing electrons causing voltage spikes.
[1] http://www.rfcafe.com/references/electrical/ew-radar-handboo...
[2] http://www.ushio.co.jp/images/en/products/list/lamp/lamp_07_...
You should probably just admit your error here -- it's light, not radio waves, causing the effect. Sure, they're part of a common spectrum, but your statement "radio waves are light" is obviously false. It's a bit like saying "red is blue".
Edit: I misread the above, lotsofmangos was talking about wavelengths coming from the radio site of the spectrum rather than light, so he didn't get it backwards. The rest of this post stands.
As to saying that radio is light, if calculating the propagation speed of a radio wave, you don't talk about its speed of radio, you talk about its speed of light. In many technical contexts, light is one of the default terms.
Also, there would be no need for the term 'visible light' if light could not be used to describe electromagnetic radiation outside the visible spectrum. Infra-red and ultra-violet are commonly thought of as light and you can even create lensing optics for radio waves and for x-rays.
edit :
The word light is commonly used as shorthand for variously the whole EM band, the near visible EM band and just the visible EM band.
If you are talking about the speed of light, it includes radio and gamma.
Discussing the colour of light on the other hand and you are referencing the visual system.
As soon as you start talking about visible light however, you are widening the definition of light again to include IR and UV and more, otherwise you would not need to clarify with the word visible.
edit 2 - darkmighty. I am not talking about the xenon bulb producing an RF EMP from the lamp flashing circuit, I am merely continuing the terminology used elsewhere in this thread, that a flash of visible light is also quite clearly a form of electromagnetic pulse.
edit 3 - foobarbecue. Radio as it gets shorter becomes microwave, then IR, then red, through green, past blue, goes to UV, then xrays, then gamma.