They didn’t die of the only ones that survived!!!
Look on page 11 of chapter 6 when the pharmacist dudes introduce themselves they are the main couple from “I’m Banging My Rival In A Parallel World” (Sen and Kasai)
Because they have the exact same names and look the exact same (but aged up obviously).
Idk if it’s the exact same couple we follow in “I’m Banging My Rival In A Parallel World” because this could be another parallel world where they are also together as a couple.
And in the latest chapter (chapter 11) when the mc’s in this find the photo they give it to Sen(pharmacist with glasses in this and Seme in “I’m Banging My Rival In A Parallel World”) because it’s a photo of him and Kasai(pharmacist with the long hair in this manga and uke in the other series) when they were younger, which is why Sen blushes. I don’t think he’d blush over a photo of his dead colleagues.
I'm still sad that the other two friends from "I'm Banging My Rival From A Parallel World!?" died here ヽ(;へ;)ノ