He's referring to comparing ground items purely via keyboard.
The comparison window only opens when you directly select a new item while the info for one is already showing, and it's currently impossible to access the keyboard look mode while an info window is open.
You can compare ground items via mouse, though even that is an imperfect system because you can only do this for items not covered by the existing info window.
I considered trying to enable keyboard look mode while an info window is open, but it's fraught with issues--not pretty. An alternative approach that would help both modes would be to always compare the current item to whatever was opened previously (even if the window was closed), though there are multiple issues with that: it looks bad and distracting when your purpose is not to compare items, you could more easily forget what the previous item was (adding its name would make the UI more confusing), and internally there are some logic issues regarding how to deal with items that no longer exist, or should no longer be known (but giving information )
Hm, after writing all that I suppose all the above issues could be fixed by always comparing so long as the previous item info was opened in the same turn. (I can still think of a condition under which it could be a source of confusion, so I might have to make that an optional feature, if added.)
