BPMN is a notation only for specifying business processes using a graphical notation. Whilst Flow rules in Pega can serve as models to confirm the business process, they are - first and foremost - an implementation artefact that contains configuration necessary for execution at runtime.
Assignment shapes in Pega Flow rules, for instance, contain rich information about routing and service levels (amongst many others). Connectors coming out of Assignments cross-reference the actions available to be performed and the order in which they are offered to the assignee.
Whilst Flow rules resemble BPMN notation - and it is nice that they do to help communicate a common understanding to business stakeholders - expect the focus to remain on ensuring the Flow rules are configurable to provide the rich execution capability we expect from Pega.