Not quite I don't think. For a busy database, The Heroku followers will not catch up to your upgraded database as quickly, so during an upgrade using Heroku's physical replication (as opposed to logical), there will be a time period where your freshly upgraded primary is on its own as the followers are being issued and brought up to date.
Except Heroku has an issue of your backup etc is too large (despite paying for the correct size), it would cause the replica to go down and spin up a new one, and this process could take hours.