Except that what it actually says, is: 'capable of being used with a firearm of any description', and in the eyes of the law, the term firearm is broader than we might suppose - everything from a PIAT to a blowpipe (and yes, it really does mention those). In that same bit of the guidelines to legislation from which that quote was taken, 'firearm' is described as literally any device of any description which can be converted to enable it to launch ammunition for which it was not intended, as an example of how bizarrely the legislation stretches examples of what that can mean, it gives the example of a Bren Gun being converted to a smooth bore single shot weapon (like anyone would ever try that). A more well known example of such a conversion which people genuinely did do on numerous occasions (as the tabloids delighted in reporting at the time), before they were removed from sale, was the Brocock cartridge air pistol revolver, which was very easily converted to fire .22 long cartridges with nothing more sophisticated than a 5.5mm reamer and a tweak to the firing pin, thus it was reclassified as a firearm, even though it wasn't one, but it was nevertheless covered by the mention of 'easily converted' weaponry in that legislation
So that certainly covers any airsoft weapon which was not originally designed, or legislated for, its ability to fire such different ammo. And that certainly includes ammo which detonates after it is fired, as is specifically mentioned. But even if none of that were so, any projectile which stores a charge that detonates after launch is almost certainly going to considered to be over the joule limit, since the definition of the joule limit for all weapons (including airguns and airsofts) specifically mentions the amount of energy the projectile possesses after it is fired, so the energy stored in any pyrotechnic charge of a projectile will also be counted and added to the kinetic energy it has. But even if that were not the case, consider the fact that pyrotechnics such as bangers, firecrackers, aeroplanes and all the other stuff we used to cheerfully chuck about in the streets in the Seventies, are now completely prohibited in the UK. The fact is, the law does not like pyrotechnics which can be launched (even by hand).
It's a bummer, but it's better to accept that idea that the Rozzers and Parliament will almost certainly scour the existing legislation to make it fit these things, rather than to get all excited about the possibilities at skirmishes, only to be disappointed further still. 'The Man' will undoubtedly push for a reference to them in the aforementioned document, as they do from time to time when something slips through. I'd put money on it.