I do not have a system to try this on - but first you must see if the partition is running in POWER8 mode.
If not, then the firmware (PHYP/FSP) do not recognize the ability in AIX 6.1.
From memory: SMT8 support was never added to AIX 6.1 - It did get added into AIX 7.1. In any case, I have not heard of anyone using SMT8 with AIX 6.1. As the documents you mention are from early and late 2015 - maybe there was an intent (so-called Statement of Direction) to add support for AIX 6.1 and/or the author was overly optimistic.
So: If it is possible - you will need a firmware from 2016 or later, the partition will need to be set to boot in POWER8 mode, and you will need to halt/activate to get into POWER8 mode if you are still in POWER7-compat mode.
However, I would question why you would want SMT8 mode (unless it is just curiosity). IMHO the best generic performance is achieved with SMT4 and/or SMT2 - depending on application. Still IMHO - ST (aka SMT1) and SMT8 are only for special applications that demand those extremes for optimal performance.
p.s. If you do get SMT8 running on AIX 6.1 - please correct me!