You are correct, the term polystrate is not technically a recognized geological term. The best way to describe it is a marketing term. There are many terms geologists use that describe either the speicific type of tree (eg. Cordaites), the time it was buried (eg. Paleoindian), location (eg. Joggins Formation) or how the object was buried (eg. Seismic Sedimentation). They had to make up their own word because to admit that 'evos' (or old earth creationists, or TE, or anyone that doesn't believe the earth is 6k years young) accept that Uniformitarianism accepts catastrophic events that can cause this and dismisses the argument that it can only happen by a slow gradual process. They attempt to back their opponents into an either-or corner where you must either accept that uniformitarianism only means 'gradual and slow with no variances' or you must believe in a 6k old earth. They falsely define the terms up front.
Normally I could understand this as a simple mistake caused by someone who doesn't understand the concepts or has never studied beyond their own circle of knowledge, but as I pointed out in a link above, these kind of questionable debate tactics come directly from their own "Baloney Detector". They are well aware of how to use these tactics and employ them every chance they can. Just like they warn people to look out for the "Baloney" of calling your opponent a "Nazi" yet, that is a tactic they use more often than I've seen most other groups use it.