Popper actually left a bit of room for doubt about the non-existence of ‘certain truth.’ Here is the quote:
As soon as we take objective knowledge into account, we must say that at best only a very small part of it can be given anything like sufficient reasons for certain truth: it is that small part (if any) which can be described as demonstratable knowledge and which comprises (if anything) the propositions of formal logic and of (finite) arithmetic.
Objective Knowledge, p. 75
