Adding something here regarding this issue.

In the French version of the game, "Summon Bahamut" and "Summon Phoenix" Doesn't require combat !

It's not indicated into the tooltip anywhere, but it make the behavior match with what is described.

In French, you can assume that when reaching lvl 70, your Dreadwyrm Trance, which require combat, evolve into an other summoning that doesn't require the combat to be initiated.

In english, we can see that the spell "require combat" everytime.