How to change the replication State Transfer SST to IST in Galera Config
This is not the whole truth. It is not about SST vs. IST it is about SST followed by an IST.

SST is a full sync, IST is an incremental sync between 2 nodes. 

Sometimes you have to use an SST. For example, when you sync your second Galera node with the first one the first time. Later on, when both nodes have the full data set and you take out one node and bring it in again, Galera will choose if IST is possible and if not choose SST.

You can force an SST in favour of IST but not the other way around.