even though there's enaugh hints towards the existence of raxxla already, from word of mouth. i think the evidence from the game itself can stand on it's own too. im talking about it being mentioned in the knowledge base in the codex. they seem to be using that section of the codex as a (definitely in-game content) section.
if you think about it and analyze that page, every entry mentioned in there, exists in-game in some way shape or form. and they are all interactable with aswell. the factions, the individuals, the thargoids and guardians, even the corporations. literally every single entry there, exists in-game, and can be found and interacted with. why should we suppose that only the last 2 entries (wich are the dark wheel and raxxla) are an exclusion to the rule, and dont exist in game?
also, if you factor in how fdev did a cleaning pass recently, by removing the fluff text from galnet (because they dont want people chasing false flags that lead nowhere), yet they kept raxxla and the dark wheel in the codex, gives it away that they are not just fluff. because generally people would tend to take the codex more seriously than galnet, and chasse it's leads even harder. and if fdev was even slightly serious about preventing players chassing false flags, they would have extended the cleaning pass to the codex page too. yet they remain there untouched, open for commanders to chasse.
"... And that's how i know!"
- felicity farseer