The distance is unknown[1] and if it were very far away, it actually could be a Jupiter-sized planet - I believe there was other circumstantial evidence that suggested this didn't "move" like a gas giant. I am not sure what a "Gaia proper motion measurement of the source" means [this is from the article] but I am not an astronomer and didn't read the whole article.
Keep in mind how they detected this: they looked at the gravitational distortion of light caused by the planet's influence. We are used to seeing this gravitational lensing with black holes, which is visibly obvious even to an untrained human eye. The effect exists for every object with mass, but it's extremely tiny for planet-mass objects. For planet detection without a host star, it is the best measurement we have, but it's very finnicky and doesn't contain very much information compared to spectral analyses, motion around a host star, etc.
From that paper's abstract:
> Although in practice [rogue planets] do not emit any light, they may be detected using gravitational microlensing via their light-bending gravity. Microlensing events due to terrestrial-mass rogue planets are expected to have extremely small angular Einstein radii (~ 1 µas) and extremely short timescales (~ 0.1 day). Here, we
present the discovery of the shortest-timescale microlensing event, OGLE-2016-BLG-1928, identified to date (tE ≈ 0.0288 day = 41.5 min). Thanks to the detection of finite-source effects in the light curve of the event, we were able to measure the angular Einstein radius of the lens θE = 0.842 ± 0.064 µas, making the event the most extreme short-timescale microlens discovered to date. Depending on its unknown distance, the lens may be a Mars- to Earth-mass object, with the former possibility favored
by the Gaia proper motion measurement of the source. The planet may be orbiting a star but we rule out the presence of stellar companions up to the projected distance of ∼ 8.0 au from the planet. Our discovery demonstrates that terrestrial-mass free-floating planets can be detected and characterized using microlensing.
AFAIU, the star being lensed was within the Milky Way, so they could establish some limits on the size of the lensing planet. The upper size bound in the paper is 2 earth masses.
(Jupiter mass does come up, but only in the context of a binary system - when ruling out that this planet is orbiting a star, they can only rule out a parent body larger than a few Jupiter masses.)
Keep in mind how they detected this: they looked at the gravitational distortion of light caused by the planet's influence. We are used to seeing this gravitational lensing with black holes, which is visibly obvious even to an untrained human eye. The effect exists for every object with mass, but it's extremely tiny for planet-mass objects. For planet detection without a host star, it is the best measurement we have, but it's very finnicky and doesn't contain very much information compared to spectral analyses, motion around a host star, etc.
From that paper's abstract:
> Although in practice [rogue planets] do not emit any light, they may be detected using gravitational microlensing via their light-bending gravity. Microlensing events due to terrestrial-mass rogue planets are expected to have extremely small angular Einstein radii (~ 1 µas) and extremely short timescales (~ 0.1 day). Here, we present the discovery of the shortest-timescale microlensing event, OGLE-2016-BLG-1928, identified to date (tE ≈ 0.0288 day = 41.5 min). Thanks to the detection of finite-source effects in the light curve of the event, we were able to measure the angular Einstein radius of the lens θE = 0.842 ± 0.064 µas, making the event the most extreme short-timescale microlens discovered to date. Depending on its unknown distance, the lens may be a Mars- to Earth-mass object, with the former possibility favored by the Gaia proper motion measurement of the source. The planet may be orbiting a star but we rule out the presence of stellar companions up to the projected distance of ∼ 8.0 au from the planet. Our discovery demonstrates that terrestrial-mass free-floating planets can be detected and characterized using microlensing.
[1] https://arxiv.org/pdf/2009.12377.pdf