I never said Grandpa is not a Soul Cultivator thoughLook, let me explain:-
1. MC's grandpa is a soul cultivator without a doubt, he can jump through bodies just like six desires dog.
TW said he was an honorary disciple of the Fate Unraveling Sect so maybe he learned both his Karma Dao and Soul Cultivation from there. He couldn't have learnt it from Six Desires cause soul cultivators have strict binding secrecy vows and also because Six Desires only recently came into the picture. Lady Butterfly is buns at soul cultivation.2. Who do you think he learnt it from? Probably both six desires dog and lady butterfly,
That's not soul cultivation sh*t though, she has passed through nascent soul and also possesses an immortal soul. If an Immortal Lord didn't even possess the ability to do this basic soul stuff and they should genuinely just kill themselves.also, all of what lady butterfly does is looking at our soul, reaving souls, and other soul cultivation ****.
In Soul Cultivation, Six Desires is a Nirvana Radiance Level Cultivator so definitely stronger than LB.3. This is the most important revelation and conclusion, at least in soul cultivation, six desires dog may be higher, more talented, or even stronger than butterfly.
There's also the chance that existing as a soul only may have weakened or restricted him, this world has a hard cap for how powerful one can be after all.