Reading that letter I think it's actually more clear what the intent
Down below it says
if you in possession of an air rifle, not air pistol (short air weapon), with a muzzle energy greater than 12
That is to say the HO see a difference between an air rifle, and an air pistol (short air weapon)... they don't regard air rifles as short air weapons, despite what the guidance says, it's only a definition for what they can see as
short air weapons. Ie they would look at a rifle and despite it being 30/60 they would see it as an air rifle, not a short air weapon. It's only in the case that someone had something that looked like a short air weapon, that then they would get the ruler out.
It's very much like capable. It's not right, it should be better. If you want to stick your neck out and throw yourself at the words rather than the reality, be my guest. But the reality is that if this has been around for some time, and BASC etc are only going to quote the guidance. They're not going to say no court is going to see an air rifle such as an HW97k as a short air weapon, because the reality is, no court ever would, or has even.
Similar thing in Germany to do with silencers, they're banned on firearms, but allowed on air rifles. Over 6ft-lb it's considered a firearm by the law, but there's loads of HWs out there with silencers on (there isn't actually a definition of a silencer). But the Germans do the same as we do, you won't find a body publicly stating a different interpretation of the law, but 1-1, verbally the situation is very different. Both here and there the authorities are aware of things that don't comply for various reasons, but they aren't looking for compliance from the top down, they are looking for compliance bottom up if you're nicked. And you aren't going to get nicked going about your normal shooting business. You're going to have to have created some sort of a stink for someone to dig out their ruler on a TX200HC, the average bobby and dare i say it FEO isn't going to be aware, and even then, you're going to have to find someone really wanting to press a point to convince a judge that the rifle he is looking at, several feet long, same length as another rifle, is actually not