Well, I'm late to this and with the end in sight, I can assume this leads to no drama. But, I think I kinda understand what author tried to convey, though. Author tried to tell that they have a very trusting relationship that he can be comfortable about sending her off to be with her ex and first love knowing nothing will ever happen. I know that feeling personally and I can feel it through their conversation when Meguro told Koga about what she felt meeting Zaizen.
But again... had I not seen the end in sight, I wouldn't be able to shake the concerned feeling off, too. Like, who knows if I just reading into that on my own because of my own bias? Yeah, I can also see why other might be concerned.
(Like, with overall tone, NTR wouldn't happen, but drama? no guarantee of that)