No, this is not what I meant.
Let me explain it on a simple example. You have a table name t1, which has 100 rows, and all PXC nodes have these identical 100 rows, there is no data difference. But, it does not mean necessary that the file t1.ibd is identical on all nodes. Each server may have it’s own, unique, binary representation of the data on disk.
Even if .ibd file is indeed identical, very likely there are some binary differences in the shared tablespace - ibdata1. But it is perfectly fine, it does not mean some node is better or worse, and if data is logically the same on all nodes, you may take backup from any of them and it will be valid.
So, when taking binary backups (and Xtrabackup does that), you need to be consistent. If you took initial full backup from node1, you cannot take incremental later from node2, it also has to be node1.