This is my view on it. Never talked this through but i think it makes sense.
As a kid the buddha relaxed and let go and gently let go into the first jhana which was a pleasent experience. He moved on because life gets in the way and the significance was not known. Then when he was practicing as an asthetic he used brutal practices to force himself into those states. It was dangerous because you torture the body until you let go. You still let go and experience jhana with those methods but it never amounted to an understanding or transformstion. Then he remembered that as a kid he relaxed into a jhana through a gentle peaseful method and he thought, maybe thats the way to nibanna. Let go and relax. Not push and force. So he made sure he was healthy and had a full stomarch, looked after himself and nurtured his way into those jhanas with surrender and relaxation with an understanding that it was safe and blameless.
I think it was the attitude of mind and the method that he recodnised as the revelation between his time using asthetic practices to let go and absorb into jhana and when he recognised that the middle way is the true way to liberation.
If I’m recalling correctly, Bhante @Sujato makes similar observations. Sometimes it is argued that the sutta is speaking about the difference between jhāna and the formless attainments, but your reasoning sounds more plausible to me.
It seem plausible to me too. Still not sure but it makes sense from that perspective. Im new to Sutta Central but I really like it because it feels fresh and light, somewhere you can relax. Other places have a rancid atmosphere where you are hemmed in and can’t open up.
The suttas are a self-contained system as has been proven over the centuries, and the fact that apparently some agamas don’t agree with the Satipatthana sutta goes to show that if a practitioner tries to implement an eclectic practice it won’t work.