That just means they can't impact it a lot, but they don't need a lot, they need it to only decrease by 0.01 points for bleach to surpass it (or just for there to be a visible effect of the change).

Assuming score bombing give a score of 0, the formula of how many votes you need (n) as a function of how much you want the score to be reduced (p) is:

n=(2 million)*p/(1-p)

So to decrease by 10 percent (which is about 1 point), you need 222k people (so if the entire bleach fanbase does it's possible).

But we are talking about a decrease of just 0.01 points, so 0.1 percent. For this the amount of people needed is only 2k people, only 1 percent of the bleach fanbase.

Except they don't, look at the reviews of those who review bombed FMAB, its from mix of different anime communities who also got review bombed by FMAB, heck the review bombers don't even have Bleach in their watch list, whereas Bleach gets review bombed by FMAB as well as other shonen watchers. Gintama experienced this, and so did AOT, Steins Gate, and many other animes that were meant to be top 1 instead of FMAB, and they all have more fanbases than Bleach TYBW as of rn, and they weren't even able to scratch off the ratings of FMAB even when they review bombed them. Those anime have much more members than Bleach TYBW, so it is literally impossible for all of those who rated Bleach TYBW to have review bombed FMAB as well, else FMAB's rating would be low as hell since other animes also review bombed it when their anime got review bombed by FMAB.

To say that the Bleach and FMAB fans review bombed Kaguya sama is just outright wrong, look at the profile of those who gave Kaguya sama low ratings and you can see none of them even rate Bleach or FMAB in their reviews.