Perhaps I am missing something because to me, going 18 years without cancer after having cancer and going through treatment is...expected? Does canceer treatment not actually get rid of the cancer or something like that?
Its no different than taking a pill for high cholesterol you are not cured of the disease .
When illness originates outside the body, it's usually possible to completely kill/eradicate it, at which point you're "cured." But some things either linger forever (certain viruses, like how chicken pox can come back as shingles) or originate inside your own body (cancer, lupus, rheumatoid arthritis). In the latter case, treatment makes it possible to return to a baseline normal, but there's no guarantee it's all the way gone or won't ever return, so we say it's in remission rather than cured. Cancer lurks; autoimmune diseases go dormant but don't die.
(One autoimmune exception: if you have ulcerative colitis, completely removing your colon cures most people, since the problem lies in that specific organ. But that goes to show how drastic a true cure is!)
There are also some conditions, notably MS, that are described as "relapsing/remitting" because they alternate between periods of relapse and remission. So you'll feel fine for a while, then the symptoms come back, then they go away again, etc.