I agree with your confusion re "one mans truth is another mans propaganda"
I think the alternate form, " one mans truth is another mans lie" works better. But this statement has issues too.
Because not all propaganda is lies. And at the same time, truth is supposed to be absolute.
And I think with subjects such as knife crime, the truth often can't actually be known.
Yes of course, police data etc is true, and is published, but we can never know for sure how well anti knife crime campaigns work. For sure, a campaign can show a reduction in recorded data, truth, but there will always be interpolation required for things such as lives saved by a campaign.
I am not referencing any PP here when I suggest this, but it's this interpolation part that conspiricists latch onto. They say "you do not know for sure, you made it up, you guessed, so it is not true"
My head hurts now. But seriously. propaganda is an armchair interest of mine. I have also debated a lot of flat earthers online, and the " one mans truth...." line is a classic gaslighting technique.