Is Yiguandao teachings part of buddhism?
Based on the wikipedia page on Yiguandao, I would say that this is not Buddhism. It seems to be a Chinese folk religion.
Why do they use Maitreya Buddha in their teaching? Well, Chinese folk religion tends to integrate elements of Taoism, Confucianism and Buddhism.
That's not surprising. Even Hinduism has the Buddha as an incarnation of Vishnu. So, it's no surprise if Chinese folk religion includes Maitreya Buddha.
But as a whole, it doesn't appear to be a Buddhist sub-school or tradition, unlike say Nichiren Buddhism.