If the accused admit to something it's more believable than the alternative (that they were forced into false admission).
So in this case if the Ukrainian government admit to making things up then I would think it's believable that they made something up for the sake of propaganda. We can also check more independent sources --read Japanese news, or Indian news sources, etc.
"It's confirmed!", "They admitted it!", and other unprovable internet turns of phrase in comments sections are really just "I believe it because somebody I trust said so" and that only has weight as long as trust has weight.