alwayson read only routing
No, it means connections with applicationintent:readonly will be routed to the readable replica. If one of them tries to modify data it will get an error message. Something like below:


  Msg 3906, Level 16, State 2, Line 2
  
  Failed to update database "XXX" because the database is
  read-only.


Note: if the secondary readable replica is not available the connection will be able to be established to the primary one depending on Connections in primary Role option.