You seem to have a small difficulty with the concept of “evidence.” Let me help.
“Evidence” is not necessarily conclusive. What Mr. Keen has posted is indeed “evidence” that his tweets are being manipulated and censored. There may in fact be an innocent explanation for his objective observations. IOW, it is not “proof”. But it is still “evidence”.
Without “evidence” which is not conclusive, our legal process grinds to a halt. The vast majority of court cases are concluded without video of the perpetrator committing the crime. They are resolved based on a preponderance of inconclusive evidence which in the jury’s mind, proves the matter (in a criminal proceeding) beyond reasonable doubt.
Thus, your complaint is irrational. Mr. Keen provided evidence of biased behavior of the part of Twitter. Others in the thread then speculated that there could be a larger movement afoot, which in some venues could be viewed as a conspiracy theory, but considering the groupthink present in the “new tech” companies like FB, Google, and the like, provides a reasonable basis for speculation.
Hope that helps.