RTFM and it all becomes clear (IMHO)
Now when you want to add a new disk type or node type, you can add it to the _existing_ cluster so long as you add a minimum of three nodes of that type. You still have a single filesystem and you can still access any file from any node. If one pool of storage fills up, provided you have not unchecked the box, the data will spill over into another pool. It's up to you where you tell the cluster to put the files and when, and it's also up to you to make sure you have enough capacity in that pool if you don't want it to spill over. It is still one cluster, not many, and it is still managed as a single cluster from a single GUI (not click 'n launch). You cannot build a new cluster and link it because that would be two clusters and you can't join two Isilon clustered filesystems together.
Even if you're not using SmartPools for tiering, you can use it to improve your data protection. In a large cluster of 80 nodes, for example, you might like to set protection to n+3 (or more) as the likelihood of another node failing whilst one is down is is higher than in a small cluster of 20 nodes. With SmartPools you can split it up into four pools of 20 nodes and set the protection level to n+2 (per pool), which would use less space for protection and give a higher level of protection.