Great questions! The framework is based on some great existing academic work from Eelco Visser's group at TU Delft. Their framework is called “scope graphs”: https://pl.ewi.tudelft.nl/research/projects/scope-graphs/

We extended scope graphs to have the symbol stack (described in OP) and also a “scope stack”, which allows us to support the more advanced examples that I alluded to at the end. So we chose the name “stack graphs” because it was “scope graphs but using stacks”.