If a tree lives on average for 200 years, and has a mortality rate of 100% in a forest fire, you don't need fires to be very common for adaptations to fire to be worth it. Just one fire in each forest every 2,000 years or so would be enough. 20,000 years should do too, but I'm less confident of this.
Was lighting more common? Probably not. Were forests bigger and less gardened to prevent spread of fire? Definitely. So for each tree, there was a much larger area of vulnerability to lightning strikes.
> Were forests bigger and less gardened to prevent spread of fire? Definitely. So for each tree, there was a much larger area of vulnerability to lightning strikes.
May be. We can't be sure about that.
Those forests housed a lot of megafauna. I can easily imagine American mammooths "gardening" the forest and removing everything green and edible from the soil. Giant slots, giant rhinos like Paraceratherium, Giant marsupials like Diprotodon, elephants and other mega herbivores most probably roamed searching the huge bulk of plant products that they needed each day. They most probable needed to complete its diet with suboptimal food like fallen leaves, barks, branches and dry weeds to reach its quota.
Most of that megafauna went extinct by men and that can't be fixed. Of those that survived, the 60% of the extant big herbivores are in danger of going extinct now, so the problem could take a big turn for worse.
European bison was a candidate to reintroduction for the possible benefit of cleaning branches, thorn shrubs and flammable materials from forests soils that cattle don't touch.
This is certainly true for the American West. Unfortunately in places like Patagonia the lenga trees never had to adapt to natural fire, so man-made fires are catastrophic.
Was lighting more common? Probably not. Were forests bigger and less gardened to prevent spread of fire? Definitely. So for each tree, there was a much larger area of vulnerability to lightning strikes.