I think it's unlikely DPC would unintentionally reuse those particular assets in this way. He could still be doing it to mess with us and they won't turn out to be Olivia and Matthew, but I think it's more likely they are and DPC is just giving sharp-eyed players a hint of a future plot point (in a way that won't actually spoil the eventual reveal).
As far as Matthew and Bianca being a couple, while the game never makes this explicit it certainly implies it very strongly. First, Jill says the other guests will be her friends and "their partners." Unless we think this is a very unusual business meeting, that suggests the others are couples (especially given how they react to the news Jill is bringing a date).
Second, we're explicitly told Matthew and Bianca made the sushi together. That would be very odd if they were just friends (unless everyone brought a dish, but that was clearly not the case). The obvious implication is that they live together. I suppose they could be brother and sister, but that goes against Jill's "partner" comment.
Third, I think the "I love you too" line goes further than you suggest. We're explicitly told that Andy was once a newcomer to the group the way the MC now is, so he must have been introduced via someone else. Olivia chiding him about never shutting up once you get to know him certainly suggests she's more familiar with him and thus is the one who first met him. In that context, it stands to reason the two were dating one point. Could they have been strictly platonic friends? Yes, but again, that goes against Jill's "partner" comment.
Finally, the seating arrangement has Matthew next to Bianca and Andy next to Olivia, just as the MC is next to Jill. Combine all that and I think it's clear why people treat the four as two couples. Is it possible DPC was trying to mislead us? I suppose, but I don't believe it. I think it's much more likely that the situation is exactly what it appears.
Which doesn't necessarily mean Matthew and Oliva are cheating, it's always possible they're in open relationships or some such thing. But I'm going to take it as read that Matthew/Bianca and Andy/Olivia are couples without some clear indication to the contrary.