Yes, you are correct. According to NEC Table 250.102(C)(1), for a 400 amp service with parallel 3/0 copper conductors, the minimum size of the grounding electrode conductor (bonding jumper) is #2 copper.Your reasoning is also correct: the table specifies the size based on the largest ungrounded conductor. Since your largest ungrounded conductor is 3/0, and the table dictates a #2 for anything over 3/0 up to 350 kcmil, then #2 is the correct choice. Flag for review