Fujito-P: "Don't worry, we may also add Dark Matter hahaha
This is clearly a mistranslation and it's referring to Murky Matter - we got Alabaster Matter from login points, but "don't worry we're also adding the other one".
I've unfortunately been busy and haven't watched the full stream, but I always like to double-check supposed mistranslations. Random redditors often have mistranslations/mistakes, so the suspicion is warranted.
First, here is the transcription of the exchange with the relevant bits highlighted (the screen doesn't show the speakers and I forgot the other dude's name, so I'm just going with A-san for the guy speaking to Fujito).
From 22:19...
A-san: こう、月替わりとかで、
別のマターが出たり...したり...
EN: So, when the month changes,
will another matter, you know...appear?
(at this point they all kind of laugh and overlap with interjections at the probing question)
Fujito: まぁ、
マター二種類ありますからね。ダークマターがくるかもしれないですね。
EN: Well, there are two kinds of matter. Maybe Dark Matter will appear.
(literally he says "Maybe Dark Matter will come" but in this context it means appear/be present)
So in this case it isn't a mistranslation, Fujito jokingly suggested adding "Dark Matter" after saying "well, there are two types of matter [in the new Limbus]." You can hear him say ダークマター (Daak Mataa) at 22:26 of the stream.
Fwiw, "Murky Matter" is メランマター (Meran Mataa) in Japanese.
https://wiki.ffo.jp/html_2006/39708.html
"Alabaster Matter" is アスプロマター (Asupuro Mataa).
https://wiki.ffo.jp/html/39707.html
And just to round it out here is the page for Dark Matter.
https://wiki.ffo.jp/html/23672.html
But at the end of the day Fujito was being coy. As you suggested, I'd expect to see the other matter as a login point reward eventually.
Also, after skimming the thread people seem to be confused about the DT-35% and DT-18% slide (around 1:33:48). This is just a formatting issue due to the slide's size constraints, no one was saying you need DT-53%.
They were just showing ways to reach DT-50%. You'd need DT-18% from other sources if you only have Empyrean +2 since the full set gives DT-32%, or DT-15% if you have Empyrean +3 since the full set gives DT-35%.