Agreed that there is a misleading on this picture.
Make sense, two MXs in front of the MX concentrator HA pair, then the ebgp can be utilized to Big office internal network or Datacenter internal core, and autovpn ibgp as wan fibric network :).
Let say, the front MX working as firewalls, inspect traffic for DIA and internet underlay path resilience, and this will allow to use full feature sets of the firewalls functions, and the concentrator HA pair designed for terminating the autovpn fibric connections only.
However, if the routed mode MX can keep all feature sets, then converge the front end MX and concentrator HA pair into one pair will save two front end MX or firewalls :).