I don't think it was an assassination attempt, but as OP says to send a clear message. If the Russians/KGB wanted them dead, they'd be dead
Any time you introduce a chemical nerve agent into a public place its big news... The only reason the death toll wasn't big is the rain. Rain renders VX impotent once dispersed. If it had been dry air and those around were able to inhale it undiluted, the game would have been over. This was a screw up or a desperation attempt to get them despite the rain.
Don't you think the Russians knew of this? If they wanted to make sure to kill in such weather conditions, then they would have used something else.
That's why I think it wasn't a botched assassination.........but a public message
Interesting angle and something to think about. However, I have never heard of such messages in another country. At home, yes where you can control the outcome of the courts and the narrative, in another country, they are risking far too much.
I ask what benefit would be gained by this to simply "send a message" in this way and they are arrested, the world suspects them, and, it could cost them economically and/or could have cost a well trained spy who is clearly comfortable with mayhem, his freedom, and maybe other assets at risk if he sings?
A message you speak of, would have been a well disguised guy walking up to him, starting a conversation and threatening him, or saying something to him in Russian to let him know "we know where you live and are watching you". Could have been a letter handed to him, or placed in his pocket, something in his mailbox, whatever.
I just don't think there is much gained. Both of them are unconscious, it's a reality they could die or have other serious injuries. Russian spies definitely know if they defect and flip, they could be targeted at any time. They know the rules of the Russian spy game very well, this isnt a new revelation to them. This message provides little value in my opinion unless it was for the purposes of "justice" as they see it.