Definition of signal bandwidth
'Bandwidth' is one of those words that gets applied to many different things, so it doesn't have a concrete, context-free, definition.
With a TDC, one context is the maximum and minimum frequencies that can be applied to the inputs, and still have the timing behave properly.
Another one is the reciprocal of the smallest time difference that can be resolved between the edges. Of course you need to consider the difference between resolution and accuracy here.
It's also worth considering this context when thinking about metastability in latches, that you need a bandwidth approaching infinity if you are going to resolve the difference between a data edge being before or after a clock edge by an infinitessimal amount. In fact when designing a TDC, metastability in the input circuitry is a key concern.
Yet another context is the rate at which successive readings are taken, which may be the same as input frequency, but need not be. This controls the bandwidth of phase modulation on the signal can be detected. Even if there is no deliberate phase modulation, phase noise from large offsets will be aliased down to baseband.