This may be a naive questions but is the parasitic capacitance of TVS Diodes a useful capacitance for decoupling a chip? Imagine I had an IC that asked for a 10pF decoupling capacitor and I used a TVS diode with a parasitic capacitance of 10pF, would that parasitic capacitance be an effective substitute for the 10pF decoupling capacitance or is there other phenomena at play here that would prevent that?
Maybe, but probably not. A key feature of decoupling capacitors is that they are able to quickly supply pulsed currents as close as possible to the chip - without the inherent inductance of copper planes or traces that carry the average power from your power supply to the chip.
When you are talking about picofarads decoupling capacitors, you're talking about extremely high frequency decoupling and thus extremely short and thin leads from your decoupling capacitor to the chip. TVS diodes are probably
1) physically too big to be suitable for this purpose (for this kind of decoupling even 0402 is big) as physical size will inherently create a current loop with some inductance, dampening the effect at the desired frequency and
2) will not be able to supply surge currents at the frequencies of interest, or at least will not be well-characterized at such frequencies.