false.
plate one is 2900- 2999 SR, if you were say around 2912. and if you played badly in those 13 games you can still drop. remember below diamond you have skill based SR. and as such you can lose more from loses and gain more from wins, or the converse. so long as you didnt fall to plat 4 i dont see an issue.
the max SR shift due to skill expression is 15 to 30 without winsreaking (over 5 wins in a row) on average you gain/lose 23 for win and lose, but if you preform well you gain more and lose less. and if you preform badly you can gain less and lose more. 6 matches loss with max loss is 180 and least gain for 7 games is 105 that is 75 shift without accounting for streaking. if say you lost nearly all 6 in a row you probably lost an additional 25.
if on previous rank up you were 2899 and they rounded you to be nice you can easily be 2799 and they round you down as you didnt preform as well.