Yes, that is precisely what it is saying.

It appears that, in the field of international flights at least, the deciding consideration as regards jurisdication is the location from which each individual sector departs (notwithstanding that the delay may not actually occur in that territory at all - i.e. a diversion en-route fully outside the EU might be what causes the delay, though the types of event that might give rise to a delay en-route are more likely to be "exceptional" events, whereas the type of negligent errors that should give rise to a right of compensation would presumably almost all be ones caused before departure i.e. the negligent act occurred on the territory of a member state)

And yes, I know the US DOT claims jurisdiction on all flights to, from, and via the US, regardless of the airline involved; but EC261/2004 does not claim the same scope as the US DOT does.

That's one way of looking at it....but you could also consider it as a form of positive discrimination in so far as it may encourage passengers to book with a Community carrier rather than with a third-country airline where a choice is available! (that said, with the proliferation of immunised TATL joint ventures, in many cases this won't have much effect on the carriers concerned! Delta still gets the same revenue, even if you switch from flying on them to flying on AF/KL over the Atlantic!)

However, it is probably best considered as what it is - that the EU cannot (easily) claim jurisdiction over negligent events occurring outside the EU and caused by airlines registered outside the EU; but for those airlines registered in the EU and for which it (presumably) therefore has a stronger claim as regards jurisdiction, it requires them to observe the "normal" standards even when on flights into the EU (NB: sectors fully outside the EU, such as KLM's DPS-SIN, will never qualify under EC261/2004).

In an ideal world, of course, each such third country would have its own regime which would look out for the interests of those passengers that otherwise seemingly fall through the cracks.

Again, I might be wrong, but I have rationalised it for myself as follows: If Delta operate a delayed AMS-DTW flight, then the "error" has almost certainly happened on the ground at AMS, or at least in the EU. (Even if it's something springing from, say, a delayed inbound arrival from the US, there would be a valid expectation that, with so many hours' advance notice, Delta should be able to spring into action and start rebooking/rerouting passengers to minimise the delay that will obviously ensue for intending passengers on that aircraft's next scheduled flight).

However, if Delta are operating a delayed DTW-AMS flight, then the "error" has almost certainly occurred in DTW, far outside European jurisdiction. There presumably isn't much chance of expecting the Dutch National Enforcement body to take a case in such a scenario, in either NL or in MI, as in general the event in question would seem to be without the jurisdiction of any such European body...and it might be expected that any such recourse should instead be pursued on the basis of whatever laws may be in force in the jurisdiction where the negligence occurred.

(And, of course, any non-EU aircraft flying into the EU will obviously be turning around and flying back; so even if that airline knows it won't have to compensate the passengers it brings to the EU with a delay, they will also know that there are "future" passengers waiting to board that aircraft for its return journey who will all be delayed as a knock-on result, so there is at least some incentive on this third-country airlines to not delay their flights into the EU by too much...)

Well, it certainly is something a savvy traveller probably already knows; if you're not on a European airline, and your flight into the EU is delayed, you've no recourse for any delay that otherwise might be covered.