The interview is important, but Monash says it's equally weighted vs ATAR and UMAT. With that in mind it's difficult to see how, relative to the interview cohort, a bottom 72%ile UMAT + a below-par 98.65 ATAR + a presumably top interview would edge out a near top ATAR + a respectable UMAT + a bottom interview; unless the former got some special consideration.
I know you've said there was no SES/bonus but we can't be sure. Monash gives certain bonuses without you needing to apply for/realising you got them.