that makes sense
Calculus uses the infinitesimal and builds off of this to derive integrals, differentiation, limits, etc.
the way newton derived everything was by making the assumption that dy = 0 and dx = 0 as two points on a graph get closer and closer
effectively, by doing 0.99999... you infinitely get closer and closer to 1, so we can say the limit of 1-dx as dx approaches 0 is 1.
infinitesimals where a very controversial part of mathematics for a long time since certain mathematicians didn't believe in infinitely small parts of an object, for example in the physical world, you cant divide an atom forever since matter cannot be smaller than Planck's constant. But in math, you can, since it's all in your head.
for more info:
https://en.wikipedia.org/wiki/Infinitesimal