I'm a 'unranked' intruder, but let me tell you how it happened.
My first provisional game was (as expected) in low-mid silver. My duo took me to play one and we lost. Then I waited a few months, and went alone - I won the next 5 in a row. Within 2 or 3 games, all silvers were replaced by golds. At 5 wins, 3 of the enemy team were replaced by plats, (2 of my team faked a DC to remake- genius) so following that I had my first loss into a plat2.
From my personal experience I've devised that... the rate of climb for your provisionals is absolutely insane. If this is normal, and a unranked (who is actually bronze) gets carried 2 or 3 times, they'll end up in your game, no sweat.