Again that is an assumption - you don’t think. Assumptions are at the root of this issue. There is no proof that they do not have the expertise, and there is circumstantial evidence that they got quantities of Sarin from Libya.
In the absence of an actual investigation, blaming Russia is just counter-productive.
A lot of factors can shorten sarin’s shelf-life:
Sarin without the residual acid removed degrades after a period of several weeks to several months. The shelf life can be shortened by impurities in precursor materials. According to the CIA, some Iraqi sarin had a shelf life of only a few weeks, owing mostly to impure precursors.[26]
https://en.m.wikipedia.org/wiki/Sarin