She already did, in the Tsubomi-focused Heartcatch sidequest where she called Nanoha.
Guess who
shares a VA, and guess how upset Nanoha was at that fact?
Flustered and embarrassed, rather. Tsubomi didn't think anything of it, is no stranger to social awkwardness herself, and is a fairly good listener, so she was able to put Nanoha at her ease about the incident.
Of course, Tsubomi has a tendency to adhere to the Code of Grandma on things like "secret identities" and "not telling other people things you don't want to bother them with." And her grandma's a very old school magical girl warrior type. If she was any older-school, she'd be an ancestral spirit.
So given that Nanoha told Tsubomi about Fate in confidence, Tsubomi's not going to relay that information to the Sailor Senshi unless, like, she specifically expects it to be the end of the world or something.
You mean she'll be her wife. The relationship between them is very not subtle.
That's the joke. People have noted the frequency with which authors and commentators take relationships with blatantly obvious gay subtext and say "and they were very close friends" about them, and started making fun of it.
It's a general form of the special case where people call Michiru and Haruka, Sailors Neptune and Uranus, "cousins" to make fun of the 1990s DiC dub of
Sailor Moon. Because the DiC dub tried to cut out the homosexual relationship between them, but even after cutting out some of the most passionate scenes, there was so much dialogue to dub where it was obvious that they were close that they decided to use the excuse of "they're cousins" to explain why they're living together. This was so obviously absurd that people mock it. It's a running gag.
Well yes, I imagine Nanoha is still due to laser her in the face.
While Nanoha doesn't make
all her friends this way (at least not in the crossover), she's certainly good at making friends that way.
(Nanoha and Setsuna are friends, but they never actually
fought; the DId You Swipe My Juice Box incident was revealed as a tragic misunderstanding before it was too late)