His guess appears to be correct judging by the dragon's reaction. This is not a wild beast but someone facing a magical effect such as Zenyatta. A trapped animal would respond far more threatening than a grumpy huff. Or perhaps having Genji as a student and calming his inner rage makes Zenyatta a dragon tamer expert.

"I will try to assist you. My name is Zenyatta. Please blink three times if you can understand me," he says. Then he waits. A single or double blink could be considered instinct and automatic. Three blinks have less of a chance of that. (And it doesn't require the dragon to nod in a narrow space.)