• this is a shitpost community, not a biotech publication, so immune here means the dictionary definition, not any domain-specific technical jargon, otherwise people can't make shitposts about diplomatic immunity
• lacking the receptor that HIV uses to hijack the regular immune response in order to reproduce means the regular immune response destroys it
• even in a normal person, after exposure, a lot of HIV gets destroyed by other parts of the immune system, often enough to eliminate it before an infection gains a foothold. Once an infection takes hold, it outbreeds the immune response as it's the part best equipped to deal with a large viral load that it interferes with.
• if you've got the virus in your body, but due to the lack of the receptor, it can't reproduce, then it doesn't remain viable for very long as each viron accumulates damage over time, and ceases to function once it's too badly damaged. People carrying a disease have enough viral reproduction going on to balance out the virus being destroyed.

When a normal person is exposed to HIV, it reproduces inside of them, so can then go on to expose more people, and if there's enough of it, infect them in turn (if there's a smaller amount, their immune system will normally be able to clean it up before it gets enough of a foothold). If someone's lacking the receptor, then no matter how much they were exposed to, their immune system will eventually manage to remove it all without becoming infected because it can't reproduce. If they had a ludicrously large viral load, then there's a possibility that it could be passed on before it was destroyed, but most of the ways people get exposed to HIV aren't enough to infect someone who's vulnerable, let alone infect someone else via secondary exposure if there's not been time for the infection to grow.

Even if you ignore that there's an entirely valid sense of the word immune that has nothing do do with biology (i.e. the one in phrases like diplomatic immunity), my original comment is entirely consistent with the dictionary definition of the biological sense of the word. There are probably sub-fields of biology where immunity is used as jargon for something much more specific than the dictionary definition, but this is lemmyshitpost, not a peer-reviewed domain-specific publication.