the distinction to make here is between technique and tone. a pro mix AE will understand all the technique aspects that a novice wont. it’s not enough to know what each piece of gear does. the next step is to know when and where to use them within the big picture. one is micro view, one is macro. another way of looking at it is the 2 sides of the brain. (left micro, right macro). left brain (intuition) is much more difficult to teach.. but this is the advantage of experience. these are the black arts. 
therefore, because the pro knows what should be done he’ll be able to make-do with plugins.
this is nothing like saying that plugins are “just as good” as high end analog gear. these are two mutually exclusive conclusions. for example, using a noise gate plug-in on close miked drum tracks:
sure, a drawmer DS-201 may sound better but the plugin will accomplish the task of “gating drum tracks.” an experienced mix AE will know that the finished record needs gated drums tracks to sound “official.” he’ll also know that a digital gate is better than no gate at all. (not to mention -in some ways- a digital gate is better. look-ahead is one example. or strip silence.. you get the idea.)
so this is what guys are talking about when they say a “real pro” can mix on anything. a full toolbox is the first consideration. the quality of the tools is a secondary consideration… within reason.
a real pro will have enough experience to know what should be done to get the tracks from A to B. he’ll devise strategies to work around the limitations he’s presented with, minimize the damage of cheap gear (gain staging is a huge one here), and optimize the end product… these are the kind of skills which will always transcend gear.
OF COURSE, better gear is always better. no one is denying this. but the best gear is of no use with a shitty skill set.. more choices, however, is not always better.
This matter should be considered carefully and studied diligently. — Musashi
audio engineering is a thankless job, haha. it’s either the talent or the gear, never the engineer. like Dennis Hopper’s character in Apocalypse Now, “He is a great man. I am a small man..”
to continue the metaphor, it’s more important to have A Noise Gate than it is to have a Drawmer DS-201 specifically. we want a full toolbox, not a screwdriver set and dremel tool. the way to understand when and where to use each tool in the toolbox comes with experience.
in digital’s defense, it is capable of a lot of techniques that analog isnt.. but digital has no personality. it’s strength is it’s weakness. by design it has no personality. it’s supposed to be repeatable. anyone can buy a plug-in and get that sound, whatever it is, good or bad. your sound should never be reducible to a preset.
so yeah, pro mixers can mix ITB because they know the techniques to get a good mix and can approximate them with ITB legos.
also remember, we also have the added haze of the loudness war constantly lowering the bar… these aspects work in synergy. the prevalence of ITB mixing does NOT mean it sounds better.
personally, i think a huge weakness of ITB mixing is the lack of decent reverb. the other aspect is the lack of personality… autotune, beat detective, elastic audio all have dehumanizing qualities.
the trick here is to strike a balance between right and left brain. analog personality and digital precision.