The assumption has always been that once the message is edited, it is the homeserver's job to serve the already edited message instead of clients having to re-aggregate all contributing events: https://spec.matrix.org/latest/client-server-api/#aggregations-of-child-events (note the presence of /messages in the list of endpoints where the server should aggregate things).

That being said, should is not must, and adding invocation of processReplacement() to addHistoricalMessageEvents() (or rather, the common code called from both addNew* and addHistorical*) would make sense to account for homeserver implementations with less comprehensive aggregation logic.