None of it really passes the smell test when you look at the facts:
- Putin has gone out of his way to mention this in an interview he knows will be broadcast to the West. Does that guarantee it is misinformation? No. But does it make it more likely than not? Without question.
- The USA would not have let Johnson sway Ukraine in that way unless they agreed. The idea that he and he alone was responsible for them walking away is almost as ludicrous as Putin's history lesson.
- Ukraine was losing at the time. If anything, it would have been in the wests favour for a peace deal to be signed, simply to buy time.
- Johnson (or the west) weren't offering or promising much at the time aside from 'support'. How likely is it that Zelensky would agree to walk away knowing that they were losing on the basis that they might get the military support they were desperate for at an unspecified time?
- The Bucha massacre had been uncovered a few days before which caused revulsion within Ukraine and undoubtably removed any remaining goodwill between Ukraine and Russia.
Is it possible that the west warned Zelensky that it wasn't a good deal and/or that their intelligence implied that Russia wouldn't stand by it? Maybe. But the claim that Johnson dared to or was allowed to unilaterally sway Ukraine in such a manner is the very definition of fanciful.
This isn't the first time that this has been mentioned though, either by Putin or people from elsewhere (Fiona Hill mentioned it last year). In terms of disinformation, yes there is an aspect of that (certainly saying Johnson
ordered them to fight on is wrong), but we really do not know what he said and what influence it had. I think there is quite a bit of evidence independent of Russia that suggests it was significant.
Also I think you are wrong to assume the USA would not have let Johnson do that unless they agreed - as I said earlier, the US had publicly said they would support Ukraine whatever they decided to do, and of course they (and we) were not really in a public position to reject them if they decided to fight on. If they had been annoyed at Johnson for it then we might expect that he would face consequences; of course he stepped down three months later after "his party turned on him".
I agree that Ukraine was in a fundamentally weak position at the time, but that is surely evidence that whatever Johnson said must have emboldened the Ukrainian government? As you say, up to that point there wasn't much in the way of meaningful support up to that point and it was very unlikely that they would kick the Russians out on their own.
Yet, after that meeting, they resolved to fight on. Is it really so fanciful to say that that decision might have been because of what he said during that meeting?