No, they are not the same at all, and your insistance that they are says to me you don't know how to use a roundabout.
The only thing you have right is that curved or straight does not matter.
On a freeway, your major direction of travel is ALWAYS parallel with the travel lanes, even when getting ready to exit. In a roundabout, it is not. You do not 'change lanes' in a roundabout, even temporarily. You cross lanes.
When you change lanes on a freeway you are driving at an angle to the direction of travel - albeit very small.
When you cross lanes in a roundabout you are driving at an angle to the direction of travel - but at a much larger angle than in the freeway case.
In both case you are driving at an angle to the direction of travel. The topology is the same .. only the size of the angle differs.