How does the phoenix’s Fire Form work?

The phoenix from MToF has an ability called “Fire Form”, which states:

Fire Form. The phoenix can move through a space as narrow as 1 inch wide without squeezing. Any creature that touches the phoenix or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage. In addition, the phoenix can enter a hostile creature’s space and stop there. The first time it enters a creature’s space on a turn, that creature takes 5 (1d1O) fire damage. With a touch, the phoenix can also ignite flammable objects that aren’t worn or carried (no action required).

I’m specifically interested in the latter part of the description: the ability to ignite with a touch.

What counts as a touch?

Let’s say that the phoenix is flying over a city, and decides to glide right over the city rooftops, could it tilt its wings slightly to graze the rooftops, and light the entire length it flies over? Mechanically, you could say that it stops every 5 feet, to use that non-action to ignite a roof, but why bother if its body isn’t entirely corporeal? Couldn’t it just do it whilst flying?