Every therapy works for someone spectacularly. This has been the case forever. It doesn't really prove anything/ main challenge is figuring out why it worked for this particular person when it fails for most. Cancer treatment has always been primitive throw shit at the wall and see what sticks. Still the case today.
If the therapy had not worked would that be evidence that the therapy is not effective? If so the fact that it did work must be at least some evidence that the therapy is effective. Simple Bayes factor, you can't have one without the other.
Additionally, if there is enough similar cancers we can know the rate of spontaneous remission within a particular band of time. This would be much lower than spontaneous remission at any time so that a spontaneous remission at the specific time of treatment could be a rather unlikely event and start to look like decent evidence.
Not disagreeing that cancer treatment isn't a shit show but I would suggest that such evidence cannot be easily dismissed due to the lack of the gold standard large scale double blind study.
You are partially correct. But here a broader picture - you may have to get in touch with the person(s) who is claiming the cure to draw a conclusion. Sometimes they may have done multiple trials within their own circle, or if it's a physician on his patients. They generally will have documented their patients. They generally will not have the means to do large scale trials. (I heard that it cost about 10 million, and about 10 years to get an FDA approval). When a unknown/non credentialed practitioners or a physician claim something it is generally ignored - as far as I'm concerned the cancer treatment cartel industry is real (As with almost every other industry), if by the rare chance it does get some publicity (as it happened with the Linus Pauling, Nicholas Gonzales), the cartel indulges in accusations of quackery and character assassination.
Now I'm not saying that the small time practitioners are always correct, in fact they are mostly wrong. But they deserve some investigation.
