21st century people were idiots. They treated cancer with poison.
For reference, "antibiotic" means "contrary to life".
We also treat cancer with surgery, the only problem being that, like the poison, it doesn't really work.
And cutting the bad parts out, can indeed work, but maybe future generations find a way to help the body stop going wrong, so you don't need those radical methods anymore.
This is much like saying that criminals are not living organisms, just parts of a society that have gone wrong.
Cancers are living organisms, similar to, but quite distinct from, the animal they developed from.
A parasite is its own organism, has its own purpose. Even though it is against the purpose of the host. Cancer has no purpose. It is a "bug" in the organism that can be fatal.
Were primitive hunters "idiots" for using crude (only by modern standards of course) sharpened sticks/stones instead of rifles?