it was naruto who summoned SO6P and it was all happening inside of naruto's brain. It had nothing to do with kamui dimension.
Maybe you may mis-understood what OP meant.
Naruto is in Kamui dimension as Obito wanted to transfer all the bijuu inside Naruto.
So this mean that Naruto is in Kamui dimension, but..
In order to summon Hagoromo, when all the bijuu + Kurama were inside Naruto, this in effect summon Hagoromo.
Hagoromo has include his own chakra into Kurama, as was mentioned before.
With all the bijuu chakra + Hagoromo chakra (part of Kurama chakra) + Hagoromo's 2nd son; these will summon Hagoromo.
Technically, the title is correctly put
If Naruto was in Kaumi dimension..
BUT
Since all the bijuu were inside Naruto, naturally Hagoromo could appear within Naruto mind (which is also can consider as another dimension).
As naturally, one part of Naruto is always in Naruto mind (dimension), if you notice whenever both Naruto & Kurama communicate to each other.
Hope this clear to you what was stated.