This has to be true, because if you can't (internally) detect the difference between nothing and a planet being nearby, you obviously also can't detect how massive the planet is, so you can't know if your acceleration relative to the nearby environment is due to gravity or something else, or even how much "absolutel" acceleration you have.
Standing on earth (or the floor of your rocket) feels different due to electromagnetic effects of the nearby "touching" external objects.
https://www.npl.washington.edu/eotwash/equivalence-principle
This has to be true, because if you can't (internally) detect the difference between nothing and a planet being nearby, you obviously also can't detect how massive the planet is, so you can't know if your acceleration relative to the nearby environment is due to gravity or something else, or even how much "absolutel" acceleration you have.
Standing on earth (or the floor of your rocket) feels different due to electromagnetic effects of the nearby "touching" external objects.