There's a couple of different ways you could continue the discussion, but I'll admit most are somewhat contrived.
1. Prime Fiction
You actually have canon that supports, to an extent, the fact that capsuleers outside of their capsule are - in general - seen as high priority targets amongst the non-capsuleer populace. The 'Jita 4-4' chronicle mentions this specifically in the context of capsuleers aboard a Caldari Navy station and how security deals with them. Translate that to your character's background as a Federation citizen and Caldari Prime as being a planet which - as far as we know is still in lockdown by a Templis Dragonaur heavy military - and you have good reason as to why your capsuleer wouldn't be able to go there.
The problem, of course, is that many players tend to ignore that piece of PF for a variety of reasons - most of them sound ones.
2. Call it as propaganda
Just like you, the other poster is unable to present canon evidence for his argument as none exists. Game mechanics do not necessarily convert directly into incontrovertible proofs in this respect. The claim that he has factories operating on the planet's surface should be taken on face value, of course - but this doesn't necessarily mean that every capsuleer can do the same ICly (even though game mechanics allows it). Perhaps the players with PI infrastructure had to strike complex agreements with the Caldari military on the planet to allow them to setup. Perhaps the PI is being utilised by the CalMil establishment to provide positive PR to the greater cluster's media outlets - *cue voiceovers from Starship Troopers* "Caldari Prime! Be all you can be, citizen! New opportunities await under the CPD's benevolent protection!
Of course, a lot of others would simply be rejected without explanation beyond "No".
The downside - if another player is unwilling to engage in this kind of RP it simply becomes a "I say, you say" circle jerk.
3. Use of background NPCs
This is a little herp-derpish, but throw a background NPC into the conversation. They were living on Caldari Prime before planet was re-taken and you were in contact for a while (after the media blackout was lifted), they couldn't provide specific details on what conditions or treatment were like (introduces the idea of censorship) but were clearly unsure what the future held. Then transmissions became less frequent, eventually stopping altogether.
Ask the other player if - given they have access to the planet's surface - whether they can find out any details on your background NPC's fate. Might stimulate some RP outside the forums in a best case scenario.
The flipside is it might be taken by the other player as an attack/chance to 'win' the argument rather than the invitation to RP that it should be framed as.
There's probably other ways to handle this, including just shutting your involvement with the debate down, but that's all I've got at present. Best of luck with whatever you decide to do.