The system offers up a bunch of "firsts" in terms of observations.
Why the dark object is only a neutron-star candidate is interesting.
The Discussion section (section 8, starting on the bottom page 12 of the PDF version) and particularly the list of possibilities in
ยง8.2 (starting on page 14) offers ways to distinguish from other possible multiple object systems.
Given more than a dozen pages describing years of observational data, effectively concluding "yet more data is needed to understand what this weird object is" is somewhat surprising, especially since (as you say) many neutron stars (and white dwarf binaries) at comparable distances have been found. On the other hand, "Unlike BHs and WDs, neutron star (NS) companions have not yet been unambiguously identified from the Gaia data".
Penultimately, if it turns out to be a main sequence star orbiting a Gaia-dark neutron star, the former's orbit reopens questions about how such binaries might form in the first place.
Finally, "dark" is in the spectrophotometry sense: the Gaia BP/RP instrument is sensitive in blue (and a little higher frequency) and red (and a little lower frequency), and the radial velocity spectrometry looks for the calcium Fraunhofer line. Neither is great for measuring a neutron star with nothing much falling onto it from the main sequence companion, and the lack of infall is relatable to the luminous star's wide and highly circular orbit. The "dark" object is likely to be loud in radio.
Why the dark object is only a neutron-star candidate is interesting.
The Discussion section (section 8, starting on the bottom page 12 of the PDF version) and particularly the list of possibilities in ยง8.2 (starting on page 14) offers ways to distinguish from other possible multiple object systems.
Given more than a dozen pages describing years of observational data, effectively concluding "yet more data is needed to understand what this weird object is" is somewhat surprising, especially since (as you say) many neutron stars (and white dwarf binaries) at comparable distances have been found. On the other hand, "Unlike BHs and WDs, neutron star (NS) companions have not yet been unambiguously identified from the Gaia data".
Penultimately, if it turns out to be a main sequence star orbiting a Gaia-dark neutron star, the former's orbit reopens questions about how such binaries might form in the first place.
Finally, "dark" is in the spectrophotometry sense: the Gaia BP/RP instrument is sensitive in blue (and a little higher frequency) and red (and a little lower frequency), and the radial velocity spectrometry looks for the calcium Fraunhofer line. Neither is great for measuring a neutron star with nothing much falling onto it from the main sequence companion, and the lack of infall is relatable to the luminous star's wide and highly circular orbit. The "dark" object is likely to be loud in radio.