I think that's how it originally worked, but then people complained that they always got support if they put it secondary.
Now the system tries a bit harder to give you your primary, at least circumstantially (e.g. you got your secondary recently, or you're playing during a popular time so the system will wait and find a primary; versus it not caring and giving secondary if you've played your primary a lot lately / are playing off peak hours). So no, it's possible you'll get top while an ally/enemy is filled support (even possible despite them queuing for top too!), Although generally I'd suspect you'll get support assigned often in that case.