They are corroborated by survivors. These are eyewitnesses of the killings, who saw their husbands or sons executed by Russian soldiers.
And if the Satellite photos are false, then the killers are still in Bucha.
The killers are quite possibly conducting the interviews, or writing the transcript (if the interview is not on camera).
Remember: the Ukraineans tortured that poor woman in Mariupol to death. They can certainly threaten a witness or compel their silence.