I don't think they store a plane eq. One texel may cover many triangles - a single plane would be an approx. and it would be impossible to calculate pixel perfect shadows.
If i'm right, all they store in the shadow map is a index to a screen pixel, and that pixel indices the next pixel falling to the same texel.
Ending up with a linked list of all pixels the texel may shadow.
Then they render a potential occluding triangle from the light position,
and for each texel it covers they check the list of pixels and mark them shadowed if they are inside all triangle planes.
No need to store planes. Seems you make a similiar wrong assumption i did initially?
Still expensive, but it might be a practical replacemant for sun cascades.
To avoid the need for conservative rasterization occluder edges could be extended in GS, allowing totally robust and accurate soft shadows too.