I worked on a major rail road in maintenance.
I have seen grinding equipment repair the rail head profile.
The sparks fly off like water from a high pressure hose.
Those showers of sparks do not ignite the creosoted ties
unless some sparks manage to get into cracks of older ties
and get to the center that may be rotted. The creosote is
injected into the ties and may not always get to the center.

Even old solid oak would be hard to ignite unless hot embers
or molten lead got to rotten parts of the wood. There would
be fumes to set off smoke alarms but there not be flames
for quite a while.

You ask, if the wood has a rotten core, why do the beams not collapse?
On the railroad there is a bridge inspector. A bridge inspection of
bridge timbers allow some degree timber rot in the center without
sacrificing the timbers strength.

I have worked above beams in old buildings and encountered thick
layers of dust. I wonder how thick the dust layer was on the church beams?
The dust could contribute to the ignition of the wood.

I am speculating on the cause of the fire from personal experience
if it was truly an accident. It is unfortunate that there is
legitimate speculation that the fire was deliberately set.