Yes… they bought multiple tickets for multiple flights out of Hong Kong, with the help of Assange. I don't think he got on any of them, but whether he intended to or not I don't know.
Assange also claims they used misdirection (open-line calls to diplomats, etc) to confuse his whereabouts.
Also note there is no footage of Snowey at Hong Kong airport. The sequence in Citizen4 is cleverly filmed to make it seem as though you're following him, but he's never in the shot.
For a good interview with Assange about his involvement, watch or re-watch the documentary Chasing Edward Snowden.