Hickman says that now..I referenced PR#5 and XMR Annual #1 because that's the last we saw the costume prior to HOX #1, and Hickman is saying he's surprised we didn't "get" the context of the costume after just the first issue
Hickman references the "most famous" time she wore it, not the most recent. That would be #137. And as mentioned, Jean didn't choose to wear the marvel girl outfit in PR#5. The phoenix created it attempting to change her mind and she immediately rejected the idea. She never "put the costume back on" there.
UXM 137 Jean wearing the MG costume was definitely more nostalgia and pride than looking into a future together with Scott since she planned from the start to end things herself if the Imperial Guard couldn't
Look at the broader context. #137 is a story about Scott and Jean. everything else is secondary. In the moment she changes into that outfit, she explains that it's a mixture of nostalgia and pride, yes- but even though she's no longer the Phoenix, she is still a telepath with a psychic rapport with scott, and whatever is coming they'll face it together. Her relationship with Scott is a central part of that identity, and his name is the last thing she screams as she kills herself. What's the most important thing to Marvel Girl? Scott is and #137 is THE issue that firmly establishes this.
After a fairly long status quo where Scott and Jean had a fraught relationship, separation, saw other people, died etc Rosenberg's run ends with Jean walking out of nowhere, and immediately making out with scott in the middle of a battlefield, reestablishing their relationship with absolutely no leadup or explanation. The next time we see her, Jean is back in her "Marvel Girl" outfit and not the X-men Red getup. Why? Hickman says to go and read #137. This one is pretty obvious here.
If Hickman wanted to callback to Jean and Scott's relationship, the 90s seem a more iconic time period.
You're missing here that Hickman is 47 years old. The 90s wouldn't be the most iconic time period for him, it would be quite a bit earlier than that and the Marvel Girl getup is his indication that he wants to return to that status quo. edit: I would also probably put "Betsy Braddock is back in her old body" in the same boat. The era Hickman is nostalgic for is not Jim Lee's 90s team.
double edit: as for the criticism of separating jean from her own story- the intent for #137 was for that to be jean herself. Everyone is aware of the retcon where that wasn't "really" her, but subsequent writers have honestly sort of just ignored this. The ANXM arc where the Shi'ar arrive on earth to wipe out the entire Grey family tree after they discover Jean has returned and put Teenage Jean on trial for things she hadn't yet done is probably the most blatant example of that.
"Jean Grey embodied the Phoenix when it obliterated 11 planets."
"Jean Grey is alive and needs to answer for her crimes."
"It is the feeling of the Shi'ar empire that it does not matter if she has not committed them yet. The creature who did it is alive and well."
"The Shi'ar empire will have Jean Grey answer for those crimes."
"The Crimes were committed. By. Her. And She will Answer for them."
The position of the rest of the intelligent races of the universe (note that the Kree, Spartax, Brood, and Annihilus are also present) is not that a copy of Jean Grey killed 5 billion people, Jean did.