This is true for "general" treatments. (Treatments applied to the whole body, EG-- "General Anesthetic") However, there are also local treatments that are more targeted that can change the environment locally.
Several such treatments exist. In the feild of cancer specifically, you have the various direct radiation treatments, the various nanoparticle+radiowave treatments, and of course, local excision treatments.
In the case of colon cancer, the inner wall of the colon has evoloved to handle some pretty extreme changes in pH, and also insane levels of salinity. Circumstances that if presented in the rest of the body would kill the patient in minutes. (If not seconds).
This tissue is also very thin, only a few millimeters thick.
The crime that the OP really has comitted is assuming that all cancers are interchangable as a general category. They arent.
In the case of colon cancer, a concentrated baking soda enema (or even a supository) would work to keep the ambient pH inside the colon quite low, and would be in direct or nearly direct contact with the cancer spreading in the colon wall at the same time. Healthy colon cells would be easily able to handle this environment, but diseased ones would not.
The major risk of complication comes from disruption of GI Flora from prolonged alterations of the pH in that environment (and from administration of probably not-very-sterile sodium bicarbonate solution directly introducing new microbial strains), and from the risk of possible impaction (if using a supository) or rupture (from overzealous enema use)
As long as the cancer has not yet metasticised, there is no real compelling reason not to couple such a clinically untested treatment like bicarbonate exposure along with a more well documented anti-cancer regimen, but some caveats apply. (If surgery was used to remove the cancer from the colon wall, then common sense applies. Dont use enemas unless directed to do so by your physician. You DONT want your colon to rupture, especially when you are immunosuppressed from having cancer.)
Just understand that not all cancers are created equally. Colorectal cancer is a very real, epidemic form of cancer. However, the ways you treat it are very different from how you would treat, eg, blood cell type cancers, or cancers in bone tissue. You dont need general application of treatment with some forms of cancer.