From the conversation it seems to be a similar situation to the project I’m with is in. The flatpak is essentially community maintained rather than being directly supported by the team. To become verified it needs to be done so by a representative of the maintainers of the software. To be verified it doesn’t have to have a team member involved in it but this is a requirement Inkscape seem to have imposed.
For us we just aren’t in a position to want to support it officially just yet, we have some major upgrades coming to our underlying tech stack that will introduce a whole bunch of stuff that will allow various XDG portals etc. to work properly with the Flatpak sandboxing model. To support it now would involve tons of workarounds which would need to be removed later.