(... snip ...)
Technically speaking you are not because you are just touching one
node of the roundabout.
Yes you are. You may not be on there very long, but you approach the
roundabout, pass the signs saying it's a roundabout, give way to those
already on it, you enter it & then indicate that you're leaving it.
Not from a data standpoint.
OSM's "lines and points" abstraction is just an abstraction of the real
world. In the real world you're on a road, and you're joining the
roundabout, staying there for a bit and then leaving it again on the
next road. Having one exit node not joined to the next entry node
better represents the real-world situation*.
* unless you happen to be riding a Spherical Cow along one of those
"frictionless surfaces" I remember from Applied Maths at school many
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