"The UUID generation algorithm described here supports very high allocation rates of 10 million per second per machine or more, if necessary, so that they could even be used as transaction IDs."
This is the use case they had in mind when building this algorithm, and it would only take about 10k machines worldwide to reach the above levels.
"Only" 10k machines producing a combined 100 billion transactions per second is pretty hard to imagine, least of all that would all be producing transactions that are part of the same namespace. Virtually all UUIDs are meaningless outside of a particular system in which they were created.
There is a solution that doesn't require extending UUIDs (which has a storage cost everyone pays), which is to use a URI/URN instead of a UUID to provide a namespace. In practice this already occurs, except the namespace (scheme, path) containing the UUID is implicit, as it hasn't been named.
"The UUID generation algorithm described here supports very high allocation rates of 10 million per second per machine or more, if necessary, so that they could even be used as transaction IDs."
This is the use case they had in mind when building this algorithm, and it would only take about 10k machines worldwide to reach the above levels.