From the article you linked:
"Novichok has, however, been known to most western secret services since the 1990s,[16] and in 2016 Iranian chemists synthesised five Novichok agents for analysis and produced detailed mass spectral data which was added to the Organisation for the Prohibition of Chemical Weapons Central Analytical Database.[17][18] Previously, there had been no detailed descriptions of their spectral properties in open scientific literature.[17][19] A small amount of agent A-230 was also claimed to have been synthesised in the Czech Republic in 2017 for the purpose of obtaining analytical data to help defend against these novel toxic compounds."
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Rate limited, so I'll reply here to the next comment:
> You'd also have to ignore the additional evidence
Ok, but if you say "only Russia have them" you're saying something that is not true. They have been synthesized outside of Russia, so it's perfectly possible that others (militaries, secret services) have them. Amounts can also have been exported from Russia in the 50 years since they've been developed.
As for the additional evidence: again, damning. However, if someone (a state actor) wanted to damage Russia, it could also have managed to attract two Russian agents in the area to frame Russia. It's a conspiracy theory of course. But we're talking about conspiracies. The bottom line is that if we conclude it was Russia then we need to ask why it would a) put the most obvious signature on the deed; b) use the most brutal and dangerous method available, one that would immediately make a vast number of very rich and powerful countries into enemies. Maybe just to sow division over the implausibility of it? I don't know.