No, that's not a speculation. In the novel version of Boruto the Movie, Momoshiki is plainly called Kinshiki'son. At the same time, there has never been mentioned such a term as "royality Ootsutsuki". They're all the same Ootsutsuki, the only thing that ties some of them like Momoshiki and Kaguya together is their arrogant attitude towards humans whom they deem "inferior beings". Yet, it's not surprising for the Ootsutsuki who are highly advanced inter-dimensional alien race.
How could they is father and son ? If Kinshiki is Momoshiki father, he shouldn't call Momoshiki just like that he is his master or sensei. Momoshiki always called Kinshiki is : Kinshiki !. He never called him : Father or Papa. I think he only is Momoshiki partner.
Drectórios, please do not double/triple post. If your comment is the latest one, remember to click the edit button on it and add to it rather than make consecutive comments within short periods of time without waiting for others to respond.
We've already told you where the info is, now if you want to go actually look, that's on you. We all already understand they're father and son, but no one just remembers off-hand which page it is. The book is available to read at various places online, go for it.
"Congratulations, it's a boy!" :-D "Your boy will grow up to be big and strong, and then eventually you'll become food for him, just as your father became food for you." :-/ In any case, you've given food for thought (no pun intended).
Edit- Oh, I just thought of something. Wasn't it presented as voluntary in the movie, but not in the manga?
In Naruto Shippuden Ultimate Ninja Storm 4 Road To Boruto, it's stated that Kinshiki is Momoshiki's FATHER FIGURE. That might mean that he's his father, or just that Momoshiki considered him his father.
Maybe it is a part of Otsutsuki clan culture for children to absorb their parents to gain their power, even though we haven`t seen many examples of it, we can guess. Perhaps this is their own twisted way of making sure that the young surpasses the old and if you think about it, there is some fridge horror in there, maybe the reason we haven`t seen Otsutsuki clan members in larger group numbers is because they attack each other for their chakra when they are not directly related( as a way of not killing their parents). The boruto movie possibly confirms it with the reveal that kaguya created the zetsu army to ward off momo and co. because they would cannibalize her.
Also this is my first post on the naruto wiki.
Here is the reason on why Kinshiki considers Momoshiki a "lord";
Due to the law of consistency, adult Otsutsukis sees the younger ones as royalties. The reason behind that Is because Otsutsuki kids carry the legacy of their ancestors. The young ones are the culmination of the continues evolution of their clan.
There is another reason why Momoshiki is "so special" to his father (to the that point he allows himself to be converted into a chakra fruit ).