Interesting. I have several thoughts on this.
First, the video you show really looks like a whiteboard and could just be another feature or type of that. You can already add a Card for an Entity, it just doesn’t show much data. If you could customize that data maybe that would do it.
That said, I don’t think it’s necessarily the best way to enable this on a global basis and make it flexible. So I’m wondering if the full drag-and-drop layout flexibility is necessary, or just one idea for how to accomplish the stated goal of “comparing entities”.
What this makes me think of is the “sidebar” (or just multiple entity windows) that some of the note-taking apps have, like Roam or Obsidian. Obsidian has a particularly good implementation where you can have unlimited columns and there is a whole set of features around managing columns, linking them together, splitting each window, pinning, etc. But I digress.
Fibery already has such a mode! It just doesn’t actually work they way I would want it to. Currently it basically puts your entity view on the side in a different single-column layout. This could handle your need, I would think, however the opened entity seems to disappear as you navigate around and I haven’t found a way to show two entities this way. The one area where the current functionality seems to be useful is in Documents (and maybe Whiteboards), where you can have the Entity View open in the sidebar and not cover your document, continue scrolling/reading your doc, and reference the Entity on the side if you want. But this too is a “fragile” state, if you click on the doc edit area the sidebar Entity view goes away.
The way I imagine it, the sidebar could do a couple of things, some of which ought to solve your problem.
First, be persistent, i.e. at least have an option that the Entity opened in the sidebar doesn’t go away if you click on the doc view, or even if you click to a totally different View, Entity, whatever. In other words, a “pin” or just “persistent” mode.
Second, you should be able to have an Entity viewed in the main “full” view, and the sidebar at the same time.
Third, there should be a hotkey that lets you open an Entity in the sidebar by default. In other words, where a normal left-click would open the entity in a pop-up, an Alt-left-click (e.g.) would open it in the sidebar.
This also suggests the sidebar should not be a “mode” of the entity view but rather its own UI pane that content can be opened into. Again this is similar to Roam, Obsidian, and others, and is very useful (as @B_Sp has pointed out, beating me to the punch! 
).
The last thing I’d like to see, although it’s not a must, is another Roam-like feature which is that the sidebar would allow multiple entities to be open at once, listed in reverse chronological order in a vertically scrollable view, and separated by some obvious delineator. You could then open any number of Entities there and scroll through them and, ideally, even collapse them. Again exactly like Roam. It’s incredibly useful there, and if “sidebar” were turned into a full UI pane and feature, it could be very useful here too.
Also I think having the Collapsible sidebar would go well with this for making more info of an Entity easily viewable on less wide screens (I am on 4k, so not so much of a problem for me )
So if some or all of that were implemented, would it meet your goals @Haslien?