Weren't the Celestial Lions the chapter they created for the RPG? It's always cool how they flesh stuff out - the Sons of the Phoenix (mentioned in the thread this spawned off of) get a whole page writeup in Vigilus.
Speaking of, here's mark II of my effortpost about marine fluff in said book. I've been reading it through the lens of a recent
interview with Phil Kelly (worth a listen - their relaunched official podcasts have been really interesting hour-long deep dives with Studio people) where he talked about how the writing surrounding primaris marines has been both intentionally created to add conflict-driven points to hang stories on and intentionally created to
not push non-primaris marines to the sidelines and to allow gamers to keep using their old marine armies without having them made obsolete by the fiction. As sort of an example, they go into some detail in the Vigilus book about how a couple chapters are (or, as the case may be, are not) integrating the primaris marines they got from Mars into their wider chapter structure, and showing some different points of view different chapters have about their new recruits.
First up was the Dark Angels, who, unsurprisingly, react to primaris marines with extreme suspicion. They basically have primaris marines operate entirely independently from "real" Dark Angels, often in straight-up suicide missions, and are in no hurry to initiate them into the chapter. On Vigilus, their primaris units (unlike the Ravenwing and Deathwing) operate more or less alongside other Imperial units and, like the other Imperial units, have no idea what the heck the Ravenwing and Deathwing are doing zipping around attacking targets and withdrawing during battles on their own recognizance. The chapter sent a primaris lieutenant as their emissary to the general Imperial war council, and eventually he straight up admits, with mostly-hidden bitterness, that the plans of the Ravenwing and Deathwing are "not for the likes of one such as him to know". So that's one end of the spectrum - chapters seeing the primaris marines as unasked for and suspicious "gifts" that don't share their chapter's culture and values, having them operate independently of non-primaris marines, which (again, entirely deliberately) allows players who don't want to use them to say "well, my chapter / the leader of this strike force is suspicious of primaris marines and doesn't use them" and have it be a fluff-supported choice.
On the other hand are the Crimson Fists, who were down to their last 50 or so marines when a whole demi-chapter of primaris marines showed up on their doorstep, literally saving their chapter from oblivion (much like my own chapter of choice, the Scythes of the Emperor). While there was some grumbling, in general the chapter leadership did everything they could to actively initiate the new primaris marines into the chapter culture, going so far as to send them off to the Fists' homeworld to take part in their blooding ritual where they hunt down a ferocious beast and slay it to earn the right to paint one hand on their armor red. So in direct contrast to chapters like the Dark Angels you have others who actively try to integrate these new guys into the chapter and its rituals and culture. So you've got chapters who could have primaris marines and non-primaris marines fighting side-by side if that's your deal.
And, of course, there are four page-long write-ups of Ultima Founding all-primaris chapters and how they're making names for themselves and carving out chapter identities even in the mere hundred or so years they've existed. All four are actually, as far as I can tell, fan-created (or at least created by GW employees as part of an informal in-house "create a chapter" challenge they had a while back), which is pretty cool as well.