Yeah sounds like it was something similar. CloudFlare showed spikes in BGP announcements (the Border Gateway Protocol - basically how each network node knows it's neighbours and the route through that neighbourhood) when the system went down. So Optus stopped telling the world it existed and everything updated around it, shutting it out from the internet.

That page about Roger's outage is an interesting read. Interac deserves a (lesser) part of the blame, such a critical service shouldn't depend on a single network provider. Good practices include redundancy in different locations and backed by different network and electricity provider, or at least not share the same local power grid.