The only people to whom that is a "competing theory" have an exceptionally high probability of being viewers of whatever the local equivalent to Fox News is.
Obviously it was Democrats who secretly deployed Novichok as part of a false flag operation.
You forgot there are people outside US? Outside its media bubble
Every place has its own media bubble. And, in many cases, those bubbles share the same conspiracy theories, especially when outside hostile actors deliberately stoke them. I assume that's why OP wrote "whatever the local equivalent to Fox News is".
You get out of media bubble by reading news from very different sources. Otherwise you're inside it no matter what you read.
I am outside the US. That is why I said "local equivalent to" Fox News (in the UK, the Daily Mail would suffice as a local equivalent).