"And I don't want to say that the Ukrainians haven't fired in that vicinity either because I think there's probably a likelihood that they have, but in good -- in a number of cases, it's returning fire of the Russians who are firing from those locations."

https://www.defense.gov/News/Transcripts/Transcript/Article/...

It doesnt seem in the slightest bit plausible to me that the Pentagon would try to justify the shelling if they didnt believe it happened.

The IAEA made no justification as to why they refused to attribute responsibility for the shelling. nor did they state in the report that they needed additional evidence to ascertain what happened. The report treated "who did it" as an unimportant detail despite knowing that it was the only reason they were invited.