The theory is that it started as a flu in china yes, but it was spread by forcing people into close quarters as they were shipped across the country.
During WW1, china wanted neutrality, but also wanted to support the allies in exchange for protection from Japan (who was an alley at the time) and a seat at the league of nations. So to have their cake and eat it too, they sent units to Canada to serve the British empire in non combat rolls. They needed to be shipped across canada before being shipped across the Atlantic to france, and they were packed into cattle cars for the journey. Very tight, unsanitary quarters, and even though thousands were falling ill, they weren't allowed to leave the confines of the train cars because the government faired there would be public outcry as racism against the Chinese was rampant in canada at the time.
So, this spread the infection quicker, and allowed it to mutate more quickly, leading to the pandemic, and spreading it to Europe.
Cases were also appearing at a military base in Kansas where troops massed before moving to Europe around this time too, so no one is for sure where the outbreak originally began.
I always found the 1918 pandemic to be a very interesting subject, and thought the loop back around to canada itt was a funny maymay, since it was brought up.