The Meltran had resolved to join or forge a nonaggression treaty with the Macross when their entire fleet arrived in Earth orbit.
Max is seen giant-sized in full combat attire, stepping aboard a Meltran battle suit, with Milia as his combat partner at the start of the final battle. He's later seen fighting alongside Milia against the Zentran. None of this could have occurred if the Meltran were willing to fight for humanity only mere seconds after Minmei began singing Do You Remember Love?
In addition no Meltran ships were seen attacking the Macross; only the Zentran ships loyal to Gorg Badolza. Interestingly enough, this means Max was to the Meltran what Minmei was to the Zentran. Max would have been unaware of the temporary nonaggression treaty between the Zentran and the Macross during his absence, so he somehow convinced them to aid the Macross in defeating the Zentran.