My understanding is that the momentary currents that flow as a consequence of indirect lightning can be regarded as almost
ideal current sources.
That being the case, the current will flow come what may, but the greater the resistance it encounters, the greater the voltage that will arise across the resistor. That is why higher resistances generally result in higher damage. For example a discharge from a cloud to a church spire’s lightning conductor creates a huge voltage and makes a big bang whereas the corresponding current down the spire’s earth conductor is the same current, but creates just a tiny voltage along the length of the conductor, so passes unnoticed.
If the firebrick has a half decent (normal household earth) connection to ground,
and has protection devices to provide a safe discharge path from the inputs, that would explain why it survived undamaged.
A modem with an earth connection, but without the input protection devices, would more likely follow the model of ‘cloud to spire’ with a very big flash and bang.
Repeat... just my understanding.