How AlwaysOn Availability Group secondary replica catches up with primary after secondary server long downtime
There is no 'cache'. Is just log. The primary log is the messaging framework. Primary writes into the log file, and secondaries receive a copy. When connected the secondaries receive the log immediately and the primary is free to reuse the log. When disconnected the primary is forbidden from truncating the log and must append to it. When reconnected the secondary receives the log it missed and primary is free to reuse again the log, no longer has to append (grow). 

You taking log backups on the primary does not change anything. When the log is required to be kept for a disconnected secondary, backup does not truncate the log.

There is no 'message' to sent to the secondaries to keep it updated. The secondary is simply running recovery on the database, that's all it does, applying the log received from the primary. The Write Ahead Protocol guarantees that whatever the secondary is 'recovering' is going to be identical to whatever the primary has in the database.

There are some optimization (eg. when connected the primary usually sends the log from memory, not from disk) and there are a bunch of additional control messages sent outside of the log stream, but these are details that distract from the core issue: the secondary is simply running recovery applying the log received from the primary, and the primary has to keep said log until it was acknowledged by secondary.

Your confusion arise from the believe that backup has truncated the log on the primary, therefore there must be another mechanism to synchronize. This is incorrect, as backup did no in fact truncate the log.