I dont think he did say Ukrainian sources couldnt be trusted. He said that using Ukrainian sources and not russian ones is unbalanced, which it is. Not wrong, not inaccurate, just unbalanced.
I dont think any of this is a matter of logic. It seems to rest entirely on who one chooses to believe. I choose to believe none of them because I know that all governments lie when it comes to stuff like this. And quite right too.
THe 'evidence' is often referrred to but never completely revealed, for obvious reasons.