You are right. That's why I wrote 'RAW'. The denim looks like it wasn't treatened in any way, but the stretch makes it different.
Nudie has some RAW denim with stretch as well. Don't know how and if the differ a lot in fabric from that.
About sizing: I think you shoudls tick to your normal Sleenker size. I had Sleenker 824p as well in size 29. Both fit the same.