Earlier quoted context omitted.
This is in line with 'guard clauses' and this is why it is accepted as idiomatic code. For this example we only have a single condition but as soon as you add more conditions it start to get out of hand. I prefer the original code because it makes the codebase as a whole easier to read. But I don't think there are any 'hard facts' to support using either of these styles over the other in these simple cases.
There is one simple hard fact: shorter code is faster to read and understand, period. If someone doesn't understand what the result of `o != Unknown` is then they should probably go back and read more about programming. Sorry to sound a bit condescending.
That's clearly not true, and one of the primary design choices Go makes. It trades concise code for more readable code.