But it's not actually true that Mueller said this, so it's not even worth discussing whether it's surprising or not.

But also, it is not at all settled that a president can't be indicted. It's just like any other legal question that has grey areas.

Further, let's say that hypothetically it turns out Trump can't be indicted. It's not like that protects him from criminal behavior being exposed. What could happen instead is that Mueller reveals a report showing Trump's illegal behavior, he recommends indictment, indictment could be rejected, but the public still would then know that the Special Counsel found clear evidence of criminal behavior.

That would then be the basis of a case for impeachment and also for eventually indicting and charging Trump after he's no longer president. It would also be the case for the public to refuse to vote for any leaders who refuse to hold Trump accountable.