69. I should like to say: “If I am wrong about this, I have no guarantee that anything I say is true.” But others won’t say that about me, nor will I say it about other people.

The first part of this paragraph is a simple continuation and re-statement of ‘Surely, we can’t be wrong about the basics? And if we are, there is no guarantee of anything else we say, whatsoever.‘

The second part “But others won’t say that about me…” is harder to grasp.  Does it mean that no one else would see our own scepticism as having such radical results? This is a likely interpretation; it matches with the inability to give any guarantee to things that you say. That is, it reduces the likelihood that things we say could become so unreliable or uncertain; no one else would ever believe that could be the case, nor would you ever believe that in regard to someone else.