It seems that only discoverFromProfile uses rdflib. Possibly it could use dependency injection and expect parser implementing RDFJS sink. Check for statement with solid:oidcIssuer can happen directly on data event.

WebID 1.0 spec has text/turtle as MUST so we just need turtle parser for OIDC Issuer discovery.

WebID requires that servers MUST at least be able to provide Turtle representation of profile documents, but other serialization formats of the graph are allowed, provided that agents are able to parse that serialization and obtain the graph automatically.