Firebreaks are a long-standing fire management technique. The problem is that with an intense fire, it's possible for the flame to leap across the firebreak, even very wide ones when there are windy conditions (which an intense fire can exacerbate).
A greater risk than flames crossing firebreaks is
firebrands or cinders - flaming bits of bark, small branch,
clumps of pine needles - being carried aloft by heat and
falling past the firebreak into combustible dry grass or other fuel, or onto a wooden deck next to wooden siding of a house, or in a rain gutter full of pine needles and twigs
next to a composition shingle roof edge.
Yeah, sloppy language on my part. I should have said "fire" rather than "flame", the former encompassing firebrands/cinders/etc.