Akin to the simultaneous fates of Schrödinger's Cat, answers to your question include no, maybe, and it doesn't matter. I have furthermore been unable to avoid philosophical diversion and hypothetical questions in this answer; please forgive me. :)
As @EricPlaton noted in comments, the answer depends on a slightly different question: What characteristics define real espresso? Here I merely compare the beverages produced by the following:
In simple point of fact, espresso is not traditionally made with a coffee-filled capsule, so the beverage produced by capsule-based machines might be disqualified from being called espresso on that trivial issue alone. :)
If you subscribe to the INEI being a benchmark of espresso production standards (PDF), capsule systems may not meet INEI parameters... so if they don't, they're not an espresso on technicality alone, with regard to "Certified Italian Espresso" at least. For example:
- quantity of coffee (e.g., Nespresso might be light);
- hot water should be used for espresso (some do use pressurized water; but it's not INEI espresso if the capsule system uses steam);
- Some capsule systems (as noted by @EricPlaton below) do produce espresso-machine pressures, but there could be benefit (or problems) by using pressure that is too high, or on the other hand too low for extraction and crema. That said, it seems that AeroPress can create crema at lower pressures, so perhaps the capsule systems produce a beverage more like AeroPress than espresso.
There have been several taste-tests of questionable "legitimacy" between capsule-based machines and traditional espresso machines. One four-subject test gave higher marks to the beverage produced from a capsule-based machine than a traditional espresso machine. This one agrees. Another test disagreed, giving a capsule-based machine a scathing review. Maybe there are better reviews out there; these are mostly useful for anecdotes.
As you note, the resulting beverage from a capsule-based espresso system is similar to traditional espresso, crema and all. However, if you ordered an espresso from a proper cafe, and they gave you a beverage that was brewed using a capsule-based machine, would you feel cheated on principle? What if the beverage was indistinguishable from traditional espresso... or if you like the capsule-based system's result better?
It doesn't matter.
This is my personal conclusion. The result of the beverage produced by a capsule-based system is a decent replica of proper espresso. Capsule systems also are likely to produce a consistent result. If you like the beverage that results (see above...), what difference does it make if it's "real" espresso or not?
 Is AeroPress espresso? I think not (and @Chris in AK agreed and notes "Aerpresso" in comments!), but that's a separate question also...