the sentence āall wcag20 statements are testableā? is a bit strong! Only a formally defined language can express sentences that can be formally, and therefore objectively, verified. And actually not even that.
The repeatability of assessments that you mention is one hypothesis; you should clearly say this; or otherwise provide evidence to support this (relatively important but demanding) claim.
I'd suggest to make it weaker, by saying that it is your assumption (and hope) that they are testable by somebody having a certain level of experience in .... and (perhaps) when following a certain procedure. No studies have been performed (yet) in order to determine is such a property is true.
We changed the language from a statement of fact 'that they ARE testable' to "All WCAG 2.0 success criteria are written to be testable".
The sentences following that sentence make it clear the person must be qualified. The qualifications to be a WCAG 2.0 tester are not formalized, and the quantification of knowledge skills and abilities to do so is beyond the scope of this document.