What with events in the real world, I'm a little zoned out, but I'll have a go... "Screenplays are structure" because the way in which events happen to your protagonist - the links between them, cause and effect, random accidents and consequences - are what shapes his emotional journey. Without the outward journey, there can be no inward journey...

I think he's saying that screenwriting is more like writing a sonnet or a villanelle than an elegy or free verse. In other words, to make the story and characters work in a screenplay, form is of the utmost importance; craft takes precedence over art.