It doesn't matter if Tendou used Gigantic's effect before the 5th summon, it's a gamble but the moment Strong has nothing to stop its effect Tendou can just Mosquito for game, I'm actually more surprise he didn't have Mosquito for this situation to just otk, but then I realize he was using Thunder Dragon package so he probably didn't have the space for it, so it's understandable he's in a losing position the moment he couldn't stop Strong's combo in the first place.
And I'm saying this even without Maxx C in place, if Strong kept the Bomber on his last turn Tendou loses either ways, Strong simply got lucky he didn't get punished because it just happen Tendou fumbled once with the Apollousa + he didn't take account that he might face a tower and can't get over it.