Again, you are just arguing semantics. The developer's rights to their property (app) specifically include the exclusive right to create copies. Depriving someone of their property rights with a dishonest intent is theft as defined through common law.
Read the law that you quoted. It specifically includes intangible property.
http://sixthformlaw.info/06_misc/statutes/16_theft_act_1968.htm
I have read the page you linked (I don't quote things without reading through them first). And whilst I agree piracy is wrong, it's also just as wrong to apply a blanket statement of "theft".
My interpretation of that law, from re-reading the page you linked:
Theft: dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it; and ‘thief’ and ‘steal’ shall be construed accordingly
Property: includes money and all other property, real or personal, including things in action and other intangible property.
So yes, in theory, theft of intangible property is possible. However, what we need to consider is exactly what the intangible is, and how piracy relates to the definition of theft. With computer software - what is the intangible property? Well, for the purpose of this debate, I'll consider it to be a) the app itself, b) the rights to distribute the app, c) the right to sell the app and d) all other rights to the app (whilst it overlaps, it allows us to explain b/c in more depth).
Now, say I downloaded a copy of the app. The definition of theft is that we are depriving the other party of something. So how does that relate back to our intangible property:
a) The developer still owns the app itself. Nothing deprived.
b) The developer still has the rights to distribute the app. Nothing deprived.
d) The developer still has all other rights to the app. Nothing deprived.
I've specifically left c out. Because, whilst the developer has received no renumeration for me downloading the app, they still have the rights to sell the app and collect payment - therefore nothing has been deprived rights-wise.
Again, this is just my interpretation and I hope I've explained my reasoning. All of this boils down to trying to apply old laws to new issues and whilst I agree piracy is wrong, I also agree it's wrong to classify it as something it's not. And again, this just covers the UK, I'm not really well read up on US law.