Its not his power, yes it is, not it isnt, yes it is...
So lets clear it up once and for all.
Naruto uses the power of Kurama.
This is naruto his power, because he obtained it(by sealing and training).
He has learned how to control the power and he has befriended kurama.
It doesnt matter if it is obtained through genetics, or by sealing.
Shinobi learn techniques. You arent born with all your techniques.
When you learn techniques, they for sure are yours right?
You can obtain power in numerous ways, training, implants, sealing,
you name it.
Sasuke uses the power of Sharingan and Especially EMS.
He obtained this power partly by genetics, partly by implant.
However, this power is Sasuke's. He has obtained it.
Jiraiya could fuse with 2 sage toads, but they were for sure part of his power.
Blood(an outside source) is required for edo tensei, but the technique is for sure part of Tobirama, Orochimaru and Kabuto their arsenal.
Animal summoning contracts need to be given to you, before you can make the contract and summon them.
You arent born with them either, but we for sure count them for a shinobi's arsenal.
Nagato has been implanted with rinnegan. Obito with Rinnegan. Madara infused himself with Hashi dna, for power ups and we all count their powers as part of them.
Danzo uses 10 sharingans, a hashi arm and a shisui eye and we all count it as his power.
Hanzo has implanted the venom sac of a salamander and we count it as his power.
I hope i have made this all clear.
Kurama is naruto his power.
EMS is sasuke his power.
and many shinobi have powers, that they not only gained by training and genetics, but through other sources as well.
We count it all as their powers.