You know instead of assuming you knew what was going on maybe you could just watch it. Time travel stories means there will be curve balls.
Even if it was Burnham it could've been:
Old Lady Burnham (Took decades for her to learn how to be a proper Mary Sue)
Made From Spores Burnham (It could happen)
Mirror Burnham (I was honestly leaning in this direction)
Mirror Georgiou (Would totally copy Burnham's brave waves)
Hyper Malevolent AI From The Future Fucking With Our Shit (We only had Airiam's word it was Burnham and I wouldn't consider her to be reliable)
Q (Because Q)
Q (Because the show needs more Suzie Plakson)
Jeffrey Combs Character To Be Named Later (WTF CBS?!?!? Hire him already)
Young NotReallyAMarySue Burnham is not any more plausible than any of these.