Your friend is both right and wrong. Right, in that you don't control what goes in the extracts, wrong in that Munich extract contains, well, Munich malt.
Most mentors will advise using the lightest extracts you can get, then adding things with specialty grains.
Think of your pale extract like a chicken breast. By itself, it tastes of, well, chicken and not much else. Think of your specialty grains as the spices that make your basic chicken breast into something interesting. If you add some black pepper, you get a different flavor. How about some Caribbean jerk seasoning? Or garam masala? Or mango-ginger marinade? That's the theory (and practice), anyway.
Now, it is possible to use other extracts to get to a certain place. Munich extracts are mashed with Munich malt only, so you get the benefit of Munich malt - a very popular ingredient in hoppy American IPAs - without having to mash it (it can't be effectively steeped). Thus, you can use pale and Munich extracts, along with specialty grains like crystal malts, to get to that place. Really, you're getting the wort from a mash of Pale, Munich and Crystal malts; you just haven't mashed the Pale and Munich - the extract manufacturer did.
Now, Amber and Dark extracts are where you lose control. Remember, all extracts start with a mash, just like advanced homebrewers start their brew days. In the case of Light extracts, the manufacturer will generally use only Pale malt. When he wants to make Amber extract, he'll add some crystal. When he wants to make Dark, he'll add black malt to the crystal and pale malts. The trouble is, you have no idea what grain he's added, unless you're very lucky and can get detailed specifications, and you can never tell how much of that grain went into the mash tun. So steeping Crystal malt to add Amber extract means you're adding more
crystal-malt stuff to a wort already fairly rich with it.
Munich extract is perfectly fine. But steer clear of extracts other than Pale/Light and Munich. And don't worry too
much about your extracts. You don't have any control over what goes in the Pale/Light extract, either!
I happen to know that Briess and Muntons both use specialty grains in their Light extracts to make them lighter.
P.S. I think your proposed Big IPA recipe sounds fine.