"The EU were effectively saying that a "defeat device" (to quote Dyson) was permitted to be used, making an energy-efficient machine appear to be less efficient than a less efficient machine (on those energy efficient labels that are mandated)."
It's worth noting that it's somewhat more complicated than that. While Dyson is correct that the tests are unfair, there are no defeat devices or cheating involved and it's nothing like the VW case. The issue is simply that before Dyson came along, all hoovers would lose suction power as they filled up. The tests they have to pass are run on a pristine machine in a clean environment, while real world use will see significantly reduced efficiency and increased power draw. Note that this does not involve cheating or detecting tests at all, it's simply that hoovers detect the conditions they're in and adjust power use to that. And as long as all hoovers worked basically the same way, the fact that real power use didn't match the tests didn't really matter because they were all affected the same way.
This only became an issue when Dyson introduced new technology that isn't affected the same way - the big selling point of their design is that suction remains the same even when the hoover starts filling up. This means that Dyson's results in tests are actually close to the behaviour in the real world, and therefore the tests put Dyson at a big disadvantage - a Dyson hoover rated at 500W will always use 500W, while a competitor might use 1kW in the real world but still be allowed to advertise it at the same 500W as the Dyson.
But importantly, note that no competitor has actually done anything wrong. Their hoovers still behave in exactly the same way they always have, and legitimately pass the tests the same way they always have. The problem is simply that tests which were flawed but acceptable for the established technology are utterly irrelevant to new technology, in a way that now puts one company at a big disadvantage. And the second problem was that the people who actually administer the tests don't appear to care in the slightest, and even said that Dyson didn't have a right to complain if he didn't do their entire job for them and design new tests that would work for everything - this new ruling specifically states that such tests already exist and that for the previous court to rule against Dyson solely due to the lack of tests rather than on the actual problem was nonsense.
So overall, Dyson is massively overstating his case by accusing his competitors of actual cheating and the EC of facilitating it. But the fundamental point of the tests being unfair and the authorities needing to be taken to court to fix it because they refuse to do their jobs is entirely correct. Hopefully Dyson will now win the original case, but I doubt he'll do well in the separate suit against the other manufacturers.
(For the record, I haven't owned a Dyson hoover since mine caught fire after just a couple of years. Dyson appears to be in the right when it comes to these issues with regulation, but I wouldn't actually recommend his products.)