So, let's say Mueller found ironclad evidence of obstruction. What would he have done? He told us that he would have said exactly what he did. He does not believe it was in his power to bring charges, only present evidence. And then he said "if we could say that the President did not commit a crime, we would have said so." He's speaking like a career lawyer because he is one; he can't outright tell Congress to bring charges.
Mueller was following Justice Department policy - he can't even say he thought a crime was committed, despite mountains of evidence, because the department's policy is it will never prosecute the president, so they can't indict, and thus won't ever accuse.
The president could murder someone on live television and department policy would be to say "Doesn't look like anything to me."
There's a pretty big excluded middle between "We can say with authority that X obstructed" and "We can exonerate X of obstructing".
(Which is not to say that details in Mueller's report don't tend to support the conclusion of obstruction, but Mueller saying that they could exonerate doesn't equate to saying that Trump did obstruct.)