ActivityStreams identifies actors by id (IRI, which by convention but not by spec is URL): https://www.w3.org/TR/activitystreams-core/#actors. This spec makes no use of username@server.domain style mentions, because those come from WebFinger. "All URIs are IRIs" (modulo Unicode), and all URLs are URIs. RFC 3986 states in reference to all but the scheme and host components:
> The other generic syntax components are assumed to be case-sensitive unless specifically defined otherwise by the scheme