Hmm ok… it's a bit of a confusing and abstract concept to close a window that isn't (visually) open in the first place, but OK I can see that there are situations where somebody wants to keep tabs in memory in the background.
However, when you have Low Power Mode selected, then this doesn't make sense to me. Why would I want to keep tabs from another group in memory if I don't even want them in the currently active group?
Having Low Power Mode selected means I only want to have the current tab (or the ones active in the last 5 minutes to be precise) alive. I think this is what the user expects. And if the user doesn't go into Activity Monitor to investigate what's going on, there could be many invisible open tabs eating up memory without the user knowing about it. It will just look like Orion isn't doing a very good job.
The Low Power Mode is main the reason why I'm using Orion. (Pages like YouTube eat up almost half a gig of memory, so tabs in the backgroud can easily eat up several gigabytes.)