Hi Ven Silence, hi others, 
I think there’s two separate questions being discussed here:
- Did the Buddha discover jhanas for himself?
- Did others practice the jhanas?
Because both could be true. The Buddha could have discovered on his own something that others were also practicing.
All we have is texts. But if there were some time machine to proof without a doubt that others practiced jhanas, that still does not say the Buddha learned it from them. I’m not saying this is true, to be clear. But to simplify the question may make it easier to answer.
I think the Buddha discovered them for himself as I explained here. But perhaps there were others who practiced them as well. The Buddha could have met them later.
Either way, just to make clear, I also think it does not really matter much.
This I think questions so many suttas that one would be putting pretty much the authenticity of the entire canon and parallels in question. The arupas are very well integrated in many texts, not just an added block of text here and there. See the Anguttara nines, for example.
Well, Thich Nhat Hahn argued the four jhanas were also additions, coming from Hinduism, so it could be worse. 