The thing is that could be true about Johnson.
It was certainly news at the time that "peace talks" broke down after Johnson's trip to Ukraine
That doesn't necessarily mean he sabotaged it though. Of course from Russia's side they're going to say that if they believe on what they want, then to their side it seems like sabotage.
Exactly what the propopsed peace agreement actually entailed is another matter altogether, and I don't imagine anyone in the public or media have any real knowledge of the in depth details of that.
I do believe Putin was being somewhat honest in the basics of that though. It was in our media at the time in 2022 that talks broke down after Johnson visited Ukraine. To suggest they broke down means at least they were talking and making some possible headway up until Boris went there.
As to why they broke down, nobody could honestly give you real answer until they see all the documents of the proposed peace agreement. I doubt it was as simple as Johnson rocking up in Ukraine and telling them to sod it and keep fighting, but that's how Putin essentially described it. The more likely scenario is the proposed agreement was heavily in the favour of Russia.
All Putin said was Johnson advised them it was better to keep fighting Russia. Let's say that's true, then why not go into details of why Johnson said that? Or why he thinks he said that? It was strange that he only gave that simple statement. If he believes that to be the case, he must have theories why that he would like to share to make his case.
He's clearly a knowledgeable man with a very good knowledge of history and was able to speak on many topics with clarity, yet he basically shut up shop on some things. No doubt an intelligent man but equally as calculated. He spoke for two hours without really saying a lot in the end.